508 
NATURE 
[SEPTEMBER 21, 1899 
fication last their lives only, or will it continue into subsequent 
asexually produced generations ? 
Let us endeavour to answer these questions :— 
(1) How will the offspring be affected? That will depend 
entirely upon how the reproductive organ was affected. Will 
the modification in the offspring have an adaptive relation 
whatever to the external cause? Now here we have a capital 
opportunity, an opportunity not afforded at all by sexual 
reproduction, of examining by experiment and observation 
the Lamarckian position. My own opinion is that there 
will be no relation of an adaptive kind between the external 
cause and the modification of the offspring. For instance, let 
us imagine, as an experiment, that a number of parthenogene- 
tically reproducing organisms are submitted to a temperature 
lower than that at which they are accustomed to live. Let us 
suppose that the cold affects their reproductive organs and pro- 
duces a modification of the offspring. Will the modification be 
in the direction of enabling the offspring to flourish in a lower 
temperature than the parent? My own opinion, as I have said, 
is that there will probably be no such tendency in the offspring, 
if all possibility of selection be excluded. But that is only an 
opinion. The question is unsettled, and must remain unsettled 
until it is tested upon asexually reproducing organisms. 
(2) Will they all be affected in the same kind of way? Yes, 
presumably they will. I arrive at this conclusion, not by ex- 
periment, but by reasoning from analogy. In the case of other 
organs of the body, the same external cause produces in all in- 
dividuals acted upon, roughly speaking, the same kind of effect, 
e.g. action of sun upon skin, effect of transplanting maize, Porto 
Santo rabbits, &c. The question, however, cannot be settled 
in this way. It requires an experimental answer 
(3) Will the modification last beyond the life of the indi- 
viduals produced by the affected reproductive organ? I can give 
no answer to this question. We have no data upon which to 
forma judgment. We cannot say whether it is possible per- 
manently to modify the constitution of an organism in this way, 
or whether, however strong the cause may be, consistently of 
course with the non-destruction of life, the effects will gradually 
die away—it may be in one, it may be in two or more genera- 
tions. There are cases known which might assist in settling 
these questions, but I must leave to another opportunity the 
task of examining them. I refer to such cases as Arfemita 
salina, various cases of bud variation which cannot be included 
under the head of growth variation. 
Senile Decay and Rejuvenescence of Organisms. 
Another question, also of the utmost importance, confronts us 
at this point. As is well known, organisms are liable to wear 
and tear, sooner or later some part or parts essential to the 
maintenance of the vital functions wear out and are not renewed 
by the reparative processes which are supposed to be continually 
taking place in the organism. This constitutes what we call 
senile decay, and leads to the death of the organism. As a 
good example of the kind of cause of senile decay, we may men- 
tion the wearing out of the teeth, which in mammals at any rate 
are not replaced ; the wearing out of the elastic tissue of the 
arterial wall, which is probably not replaced. There is no 
reason to suppose that the reparative process of any organism is 
sufficiently complete to prevent senile decay. There is probably 
always some part or parts which cannot be renewed, even in the 
simplest organisms. Maupas has shown that this holds for the 
ciliated Infusoria, and he has also shown how the renewal of 
life, which of course must be effected if the species is to con- 
tinue, is brought about. He has shown that it is brought about 
by conjugation, during which process the organism may be said 
to be put into the melting-pot and reconstituted. For instance, 
many of the parts of the conjugating individuals are renewed, 
including the whole nuclear apparatus, which there is every 
reason to believe is of the greatest importance to living matter. 
On reconsidering the life of the Metazoa in light of the facts 
established by Maupas for the Infusoria, we see that all Metazoa 
are in a continual state of fission, as are the ciliated Infusoria. 
They are continually dividing into two unequal parts, one of 
which we call the parent and the other the gamete. The 
parent Metazoon must eventually die; it cannot be put into the 
melting-pot ; its parts cannot be completely renovated. The 
gamete can be put into the melting-pot of conjugation, and give 
rise to an entirely reconstituted organism, with all the parts and 
organs brand new and able to last for a certain time, which is 
the length of life of the individual of the species. 
NO. 1560, VOL. 60] 
Is there any other way than that of conjugation by which an 
organism can acquire a complete renewal of its organs? Is the 
renewal furnished by the development of all the parts afresh 
which takes place ina parthenogenetic ovum such a complete 
renewal? This question cannot now be certainly answered, but 
the balance of evidence is in favour of a negative answer. And 
this view of the matter is borne out by a consideration of the 
facts of the case. In all cases of conjugation which have been 
thoroughly investigated, the nuclear apparatus is completely re- 
newed. It would appear, indeed, as though the real explanation 
of the uninuclear character of the Metazoon gamete is to be 
sought in the necessity of getting the nuclear apparatus into the 
simplest possible form for renewal. Now in the development of 
a parthenogenetic ovum the ordinary process of renewal of the 
nucleus is often in partial abeyance. As a rule, it only divides 
once instead of twice, and there is, of course, no reinforcement 
by nuclear fusion. It is, of course, possible that the reinforce- 
ment by nuclear fusion which occurs in conjugation may have a 
different explanation from the nuclear reconstitution which 
takes place in the formation of polar bodies and similar struc- 
tures. On the other hand, it may all be part of the same pro- 
cess. We cannot tell. So that we are unable to answer the 
question whether for complete rejuvenescence a new formation 
of all parts of the organism is sufficient, or whether a reconstitu- 
tion of the nuclear apparatus of the kind which takes place in 
the maturation of the Metazoon ovum and the division of the 
micro-nucleus of Paramcecium is also required; or, finally, 
whether in addition to the latter phenomenon a reinforcement 
and reconstitution by fusing with another nucleus is also neces- 
sary for that complete rejuvenescence which enables an organism 
to begin the life cycle again and to pass through it completely. 
With regard to buds in plants, there is reason to believe that 
they share in the growing old of the parent. That is to say, if 
we suppose the average life of the individual to be 100 years, a 
bud removed at 50 will be 50 years of age, and only be able 
to live on the graft for 50 more years. 
Heredity. 
Having now spoken at some length o. the phenomenon of 
variation, I must proceed to consider from the same general 
point of view the phenomenon of heredity. 
As we have seen, in asexual reproduction heredity appears, as 
a general rule, if not always, to be complete. The offspring do 
not merely present resemblances to the parent—they are 
identical with it. And this fact does not appear to be astonish- 
ing when we consider the real nature of the process. Asexual 
reproduction consists in the separation off of a portion of the 
parent, which, like the parent, is endowed with the power of 
growth. In virtue of this property it will assume, if it does not 
already possess it, and if the conditions are approximately 
similar, the exact form of the parent. It is a portion of the 
parent ; it is endowed with the same property of growth ; the 
wonder would be if it assumed any other form than that of the 
parent. Indeed, it is doubtful if the word heredity would ever 
have been invented if the only form of increase of organisms 
was the asexual one, because there being no variation to con- 
trast with it, it would not have struck us as a quality needing a 
name, any more than we have a name for that property of the 
number two which causes it to make four when duplicated. 
The need for the word heredity only becomes apparent when 
we consider that other form of reproduction in which the real 
act of reproduction is associated with the act of conjugation. 
Looking at reproduction from a broad point of view, we may 
sum up the difference between the two kinds, the sexual and 
the asexual, by saying that, whereas the essence of sexual repro- 
duction is the formation of a new individuality, asexual re- 
production merely consists in increasing the number of one kind 
of individual. From this point of view sexual reproduction is 
better termed the creation of a new individuality, for that, and 
not the increase in the number of individuals, is its real result. 
Inasmuch as conjugation of two organisms is the essential 
feature of sexual reproduction, it would appear that the number 
of individuals would be actually diminished asa result of it ; and 
this does really happen, though ina masked manner, for we are 
not in the habit of looking upon the spermatozoon and ovum as 
individuals, though it is absurd not to do so, as they contain 
latent all the properties of the species, and are sometimes able 
to manifest these properties (parthenogenetic ova) without con- 
jugating. In some of the lower organisms the fact that con- 
iugation does not result in an increase of the number of 
