newformed duplicate Infusoria multiplied subsequently by spontaneous division. From one Oxytrachia cut in two, Nussbaum succeeded in raising ten normal animalcules, which subsequently all encysted. After an unequal division, the parts are both still capable of regeneration, but parts without a nucleus did not survive, which suggests that the formative energy is in some way bound up with the nucleus. But nucleate pieces may break down. Thus, all attempts at artificial multiplication of the multinucleate Opalina failed, although the division of Actinosphærium had been successfully made by Eichhorn as long ago as in the last century. Pelomyxa palustris has been successfully divided by Greef, and Myastrum radians by Haeckel.

Gruber (l. c., p. 718) describes his experiments with Stentor : "If one divides a Stentor transversely through the middle, and isolates the two parts, one finds on the cut surface of the hind part, after about twelve hours, a complete peristomial field with the large cilia and buccal spiral newly formed. On the other hand, the piece on which the old mouth is situated has elongated itself backwards, and attached itself in the manner peculiar to these Infusoria. If one has made a longitudinal section, so that the peristom is cut in two, then the peristoms both complete themselves and the lateral wounds heal over. I have repeatedly separated, by transection, pieces considerably less than half of the original Stentor, and these have also regenerated themselves to complete animals." Gruber, too, observed that artificially divided Infusoria were capable of subsequent spontaneous multiplication. If the section is not very deep, there may arise double monsters; but here, just as in spontaneous divisions, as long as there remains an organic connecting band, the two parts act as one individual, showing that the nervous actions are not restricted to determined paths. Gruber also adds that two divided pieces may be reunited if brought together quickly enough. The observation thus briefly announced is of such extreme interest and importance that the publication of the full details of the experiment will be eagerly awaited. Gruber adds that at present we can

not go much beyond the proof of existence, to a high degree, of the regenerative capacity in unicellular organisms, He also makes the significant observation that in the Protozoa, we have to do foremost with changes of function; in the Metazoa, with growth also.

2. Duplication of parts.-In these anomalies we find an organ which, although an extra member, yet still conforms to the type of the species. For example: a frog is found with three posterior limbs; dissection proves the third leg to agree anatomically with the typical organization of the frog's hind leg. In determining the importance to be attributed to this evidence, it should be remembered, on the one hand, that these instances are by no means unusual; on the other, that the agreement with the normal structure is not uniform.

3. Asexual reproduction.-When a species multiplies by fission of any kind, we must assume that each part, after division, possesses the formative tendency, since we see it build up what is necessary so complete the typical organization of the individual. Again: a bud of a hydroid or polyzoon, although comprising only a small part of the body, is equally endowed with this uncomprehended faculty. In pseudova we reach the extreme limit; in aphis, for example, the parent gives off a single cell, the capacity of which, to produce a perfect and complicated individual, fully equals the like capacity of a hydroid bud or of half a worm.

The evidence forces us to the conclusion that the formative force or cause is not merely the original disposition of the forces and substances of the ovum, but that to each portion of the organism is given: 1. The pattern of the whole organism; 2. The partial or complete power to reproduce the pattern. The italicized formula is, of course, a very crude scientific statement, but it is the best which has occurred to me. The formative force, then, is a diffused tendency. The very vagueness of the expression serves to emphasize our ignorance concerning the real nature of the force. In this connection, I venture to insist upon the fact that we know little or nothing concerning any of the fundamental properties of life, because I think the

lesson of our ignorance has not been learned by biologists. We encounter, not infrequently, the assertion that life is nothing but a series of physical phenomena; or, on the other hand, what is less fashionable science just now, that life is due to a special vital force. Such assertions are thoroughly unscientific; most of them are entirely, the remainder nearly worthless. Of what seems to me the prerequisites to be fulfilled before a general theory of life is advanced, I have written elsewhere."

II. CONCEPTION OF DEATH.

My thesis reads: There are two forms of death. These are first, the death of the single cells; second, the death of multicellular organisms. Death in the one case is not homologous with death in the other.

Weismann assumed the complete homology of the two forms of death. Without this assumption, his hypothesis of the immortality of unicellular organisms falls to the ground and with it falls the entire superstructure of his speculations upon germ plasm. Oscar Hertwig (Zeit und Streitfragen, Heft 1) has already expounded, very clearly, the dependence of the theory of germ plasm upon the hypothesis of unicellular immortality; it would, therefore, be superfluous to discuss it here.

The conception of the biological problem of death, to which I still hold, was formed several years before Weismann's first publication, which appeared in 1882, with the title, " Ueber die Dauer des Lebens." He has further defended his view in his article, "Ueber Leben und Tod" (1884), and has steadfastly adhered to it since. In the years 1877-1879 I published my theoretical interpretation of the problem. This interpretation became the starting point of elaborate special investigations, by which I endeavored to advance the solution of the problem. and, in fact, observation and experiment have confirmed the

5 C. S. Minot, On the conditions to be filled by a theory of life, Proc. Amer. Assoc. Adv. Sc., XXVIII, 411.

6 Proc. Boston Soc. Nat. Hist., XIX, 167; XX, 190.

original thesis. Moreover, in an especial short article I have
directed attention to the fact that Weismann has not consid-
ered the essential issue of the problem. The difficulties
pointed out still remain, and, according to my conviction, can-
not be removed. Weismann passes these difficulties by and
carries out his speculations without first securing a basis for
them. His method is illustrated by the following quotation:
"I have, perhaps, not to regret that I cannot here discuss the
article referred to (Minot's Article in Science, Vol. IV, p. 398);
nevertheless, almost all objections which are there made to
my views are answered in the present paper." (Weismann,
Zur Frage nach der Unsterblichkeit der Einzelligen, Biol.
Centralbl., IV, 690, Nachschrift). I have studied the paper
with conscientious care and cannot admit that the objections
have been answered. On the contrary, I maintain now, as
formerly, the judgment: "He misses the real problem."
For this reason I hold it to be unnecessary to discuss the de-
tails of Weisman's exposition, because—if I am right—he has
not considered the actual problem of death at all.
"He
misses the real problem." The following reasoning leads to
this decision Protozoa and Metazoa consist of successive gen-
erations of cells; in the former the cells separate; in the lat-
ter they remain united; the death of a Protozoa is the anni-
hilation of a cell, but the death of a Metazoon is the dissolu-
tion of the union of cells. Such a dissolution is the result of
time, that is to say, of the period necessary to the natural
duration of life, and we call it, therefore, "natural death."
Moreover, we know that natural death is brought about by
gradual changes in the cells until, at last, certain cells, which
are essential to the preservation of the whole, cease their func-
tions. Death, therefore, is a consequence of changes which
progress slowly through successive generations of cells. These
changes cause senescence, the end of which is given by death.
If we wish to know whether death, in the sense of natural death,
properly so called, occurs in Protozoa or not, we must first pos-

'Journal of Physiology, XII, and Proc. A. A. A. S., XXXIX, (1890).

617

sess some mark or sign, by which we can determine the occurrence or absence of senescence in unicellular organisms.

Around this point the whole discussion revolves. Certainly a simpler and more certain conclusion could hardly be drawn. than that the death of a Metazoon is not identical, i. e., homologous with the death of a single cell. Weismann tacitly assumed precisely this homology, and bases his whole argument on it. In all his writings upon this subject, he regards the death of a Protozoon as immediately comparable with the death of a Metazoon. If we seek from Weismann for the foundation of this view we shall have only our labor for our pains. Starting from this view Weismann comes to the strictly logical conclusion that the Protozoa are immortal. This is a paradox! In fact, if one compares death in the two cases, from Weismaun's standpoint, then we must assume a difference in the causes of death, and conclude that the cause in the case of the Protozoa is external only, while in the Metazoa it is internal only, for, of course, we may leave out of account the accidental deaths of Metazoa. If we approach the problem from this side, we encounter the following principal question: Does death from inner causes occur in Protozoa? Weismann gives a negative answer to this question, with his assertion that unicellular organisms are immortal. The assertion remains, but the proof of the assertion is lacking. In order to justify the assertion, it must be demonstrated that there does not occur in Protozoa a true senescence, showing itself gradually through successive generations of cells. Has Weismann furnished this demonstration? tainly not. He has, strictly speaking, not discussed the subject. It is clear that we must first determine whether natural death from senesence occurs in Protozoa or not, before we can pass to a scientific discussion of the asserted immortality of unicellular beings. The problem cannot be otherwise apprehended. Weismann has not thus conceived it, therefore the judgment stands against him: he misses the real problem.

Cer

Senesence has been hitherto little investigated; for many years I have been studying it experimentally and have tried