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no longer exist; the single adaptive one would have replaced it. But it is obvious that the appearance of complicated details of marking and color, such as ocelli are, cannot be simply the direct effect of heat or cold, drought or humidity. These influences are not the actual causes of such formations, but only the stimulus, which sets their primary constituents free, i. e., induces their development, as I tried to demonstrate in the lecture above noted. As the sufficient cause of the sleep of the marmots does not lie in the cold, but in the organization of the animal which is adapted to the cold, and as the cold only brings the existing predisposition to winter sleep into play, so among these butterflies with adaptive seasonal dimorphism the display of the one or the other marking is apparently connected, partially, at least, with one of the above named outward influences, although in reference to these trophical butterflies we do not yet know to which of them.

We recognize temperature as the stimulus to development with the cases of seasonal dimorphism of our indigenous butterflies, as in all cases of seasonal dimorphism, which have hitherto proved experimentally, it is always high and low temperature which gives the outward impulse to the appearance of the one or the other form where this impulse did not come exclusively from within.

There are, therefore, two different sources of the appearance of seasonal dimorphism: on the one hand, the direct action of alternating external influences, viz.: temperature, can bring about this change in the outward appearance; and on the other hand, the processes of selection. It is therefore necessary to consider these two kinds of seasonal dimorphism separately. It will certainly not always be easy to decide between them when a particular case has to be dealt with, as at present it is not always possible to say whether a coloring or marking has a definite biological value or not. Both causes also may co-operate in in one species.

Note on the Classification of Diplopoda.-The admitted impossibility of formulating a generally satisfactory definition of the term species exists partly because systematists have used it in the greatest variety of applications, and partly because natural groups are so diverse in structure and developmental history that a scheme calculated to elucidate one may increase confusion in another. It is hence desirable in proposing or making use of a classification to recognize as clearly as possible the conceptions under which the arrangement into the various categories of natural groups has been made.

The structure and distribution of the Diplopoda make it advantageous and usually easy to arrange them into species, which are groups

of very similar individuals not connected by intermediate individuals with other groups different in details of structure, form or color. An apparent and probably sufficient cause for this is the close similarity of all Diplopoda in life-histories, habits and food. All are scavengers, able to subsist upon a variety of decaying vegetable, or even animal matter, and there has been scarcely any response to calls for special adaptations to life as parasites, commensals, or under other changed conditions. The species of Diplopoda are not only extremely local in distribution, but are generally confined to almost identical habitats, removed from which they do not long survive.

Supposing the Diplopoda to be a natural group descended from a common ancestor, we are compelled to believe that such differences as appear among them are the result of accumulated variation not greatly influenced by external selective causes. Hence, existing differences indicate in general much more remote developmental divergence than in groups which have entered more thoroughly into the struggle for existence by responding to the demands of varied conditions. In this respect the Diplopoda offer a most striking contrast to the Hexapoda, and the results are in accordance; there are more millions of species of Hexapoda than there are thousands of Diplopoda.

Having accepted a criterion of species, the classification into higher groups is perhaps largely a matter of convenience; but convenience, scientific accuracy, and the recognition of affinities, alike demand constant attention to the fact, that the value of any character depends primarily upon its constancy, not upon the apparent degree of divergence. This is merely the reiteration of the chief axiom of systematic science, but the abundance of systems which completely ignore this fundamental idea are evidence that much reiteration is still desirable.

While in some natural groups it seems necessary to recognize subdivisions not definable by any constant character or complex of characters in the Diplopoda, we may conveniently proceed upon somewhat better ground, and require that the genera and larger divisions shall be limited by definite structural characters.

A dichotomous classification is theoretically the only exact one, for the reason that three or more natural groups could never be expected to be separated by exactly equivalent structural differences. Practically, however, a dichotomous system is inconvenient by reason of the great number of categories necessary in properly recognizing affinities. Hence, it is not a valid objection to the usual or multifid form of classification that the natural divisions arranged under the same category are not of the same rank, that is, not remote from

each other by equal structural distances. All that can be reasonably demanded of a classification is that its groups of all ranks shall be natural ones, and that the higher the groups, the more constant, and hence fundamental, shall be the characters by which they are separated. Furthermore, it must never be supposed that the variability of a character in one group need affect its importance if found to be constant in another.

As a general policy it is evidently desirable that scientific names of all grades shall mean as much as possible. The objection to the recognition of distinct and definable genera and higher groups on account of the consequent multiplicity of names is usually to be taken as an unscientific willingness to ignore structural differences and natural affinities, in the hope of escaping additional labor. In reality the difficulty of defining groups containing unrelated members, and of becoming acquainted with such through descriptions, much exceeds the temporary inconvenience resulting from change of names.

In attempting to embody in the classification of the Diplopoda a recognition of certain structural differences found to be invariable, several natural and distinct groups of families have been recognized as orders. It is here proposed to render this classification more definite and consistent by the division of two of these orders, in the belief that the resulting groups, in addition to numerous structural differences, have long been divergent in developmental history. The orders thus to be divided are the Diplocheta and the Merocheta. From the Diplocheta it is proposed to separate the true Iulidae and their allies, under the name ZYGOCHETA, leaving under the Diplocheta Spirostreptoidea and Cambaloidea. The Zygocheta are distinct in many characters of the gnathochilarium, in the transformation of the first pair of legs of males as clasping organs, the adnate external seminal ducts, the absence of legs from the third segment, the presence of legs on the fourth segment, and the structure of the copulatory organs of both sexes. The Diplocheta have the first pair of legs nearly or quite unmodified, the external ducts distinct, the third segment with a pair of legs, and the fourth segment footless. Notwithstanding these and other important and invariable differences, it remains probable that these two orders are more related to each other than to any third group of Diplopoda.

The other case is similar; the Merocheta will, in the restricted sense, contain numerous families allied to the Polydesmidae, with twenty closed segmental rings; the new order CLOCHETA will accommodate the

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Lysiopetaloidea and Craspedosmatoidea, and is characterized by the greater number of segments, the free pedigerous laminæ, the sevenjointed legs, the distinct mentum, and the normal presence of eyes. In the Merocheta the apertures of the external seminal ducts are small openings in the chitinous wall of the coxæ of the second legs, connecting with internal tube of nearly uniform diameter. In the Colocheta the coxæ contain a large cavity, while the aperture is large, the margin pilose and not chitinous.-O. F. COOK.

EMBRYOLOGY.'

The Tentacular Apparatus of Amphiuma.-In the Journal of Comparative Neurology, Vol. VI, March, 1896, Professor J. S. Kingsley has written an article entitled "On Three Points in the Nervous Anatomy of Amphibians" in which he has endeavored to show that the tentacular apparatus of Amphiuma, briefly described by me (Jour nal of Morphology, Vol. XI, No. 2), has been mistaken for a nerve and blood vessel. I consider the discovery of this degenerate organ of too much phylogenetic importance to be consigned at once to oblivion, and, therefore, offer in this article the results of a more careful study of it.

Since histological detail is important in this investigation, I state briefly the technique. The specimen, seventy-eight millimeters in length and seven millimeters in body diameter, was hardened in Kleinenberg's picro-sulphuric and, passed through the alcohol series from seventy to one hundred per cent and returned to seventy per cent, when the head was severed and placed three days in borax-carmine, then in acid alcohol twenty-four hours, after which it was imbedded in paraffine by the usual method and cut into serial sections one twenty-fifth of a millimeter in thickness.

Figure I is magnified twenty diameters. The outlines of all the features were drawn with a Zeiss camera lucida. Every feature appears in

From the true Craspedosomatidæ there may be distinguished the Trachygonidæ, Conotylidæ, and Cleidogonidæ, in addition to the Chordeumatidæ established by C. L. Koch in 1847. The separation of other equivalent groups will probably be necessary when a fuller knowledge of European and Asiatic forms is gained.

Edited by E. A. Andrews, Baltimore, Md., to whom abstracts reviews and preliminary notes may be sent.

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Figure I. Right-hand portion of section through head of Amphiuma 78 millimeters long, f, frontal; P, parietal; OSP, orbitosphenoid; E, eye; m, maxillary bone; mx*, branches of maxillary nerve; Tt, tentacular apparatus; rt, retractor muscle; mx, maxillary nerve.

the section just as distinctly as it is shown in the figure, b is the blood vessel and the adjacent mx* the nerve which Kingsley thought I had mistaken for the tentacular apparatus, Tt. Notice that three branches of the ramus maxillaris course along the external sheath.

mx*

Figure II. Cc, canal for tentacle; rt, retractor muscle; ObD, orbital gland; ITts, inner sheath; ATts, outer sheath.

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