Fan. 25, 1883] 
NATURE 
393 
From the observations which Livingston Stone has made in 
1878 in the North American institutions for fish-culture on the 
McCloud River, it follows that 14,000,000 eggs obtained from 
ripe but relatively young and smaller salmon were without ex- 
ception at least one third smaller than the millions of eggs 
which were before obtained from older, larger salmon of the 
same species, and that nevertheless they developed quite 
normally. By these ob-ervations the fact is established that the 
salmon, when older, lays larger eggs than at a more youthful 
age, and this, more especially, is of great value for our hypo- 
thesis. Firstly, the size of the egg must influence the chances 
which they have for escaping or falling a prey to different 
voracious animals, In this respect the smaller eggs are exposed 
to other dangers than the larger ones. Furthermore, the relative 
size of the egg will, without doubt, exert a certain influence— 
however insignificant—upon the individual which is developed 
out of it. 
In comparison with the larger egg of the older salmon, either the 
food-yolk or the formative yolk in the smaller one will be of 
smaller dimensions, or both together will have been reduced in 
size. In each of these three cases, even in the last-named, the 
conditions under which the smaller egg (that is to say, the whole 
generation of the first-born) attains its development, differ from 
those of the generation issued from the larger egys, the genera- 
tion of the last-born. The first-born will either be of smaller 
size, or because they possess a smaller food-yolk they will have 
to provide their own nourishment at an earlier date; or both 
circamstances are combined. 
Nobody will deny that in each of these cases natural se- 
lection can freely come into play. In addition to this it must be 
remarked that however insignificant this difference in external cir- 
cumstances may be its presence is nevertheless undeniable, since 
it reappears again with unerring certainty in every successive 
generation. In this way the effect can gradually accumulate, 
and finally the path may have been entered upon which leads to 
a specific differentiation of the descendants of the first and of tke 
last born. 
This having taught us that indeed the external circumstances 
which preside at the birth and at the growth of the first and the 
last born are different (at least for this species of salmon, reliable 
observations on a similar scaiz concerning other animals being 
for the present wanting), I must now call your attention to the 
second cardinal point, viz., that the internal properties and 
acquirements with which each of the two series of births is pro- 
vided, are also different. Heredity has indeed invested them 
with peculiarities, part of which show themselves in their 
organisation, another part remaining latent, and only attaining 
development in following generations. Such a latent potential 
tendency towards eventual modification of the individual or his 
progeny, must needs find more numerous occasions to unfold 
itselr in the first born, simply because these are possessed of a 
larger number of ancestors. On the contrary those that have a 
smaller number of ancestors, z.e. the last born, have had this 
occasion for development offered to them at rarer intervals. 
From this it follows that further modifications under the influ- 
ence of natural selection will be started by preference in the 
different series of first born, because ceteris paribus, there are 
here more chances for the appearance of small deviations, which 
toa certain extent are always due to reversion to the parent 
forms. 
And so there is reason to suppose that also the internal 
properties of the series of first born «iffer from those of the last 
born, in the same way as we have just defined it for external 
agencies. In my opinion the difference in internal structure is 
of greater consequeuce than that in external agencies, although 
we must at the same time acknowledge that our present methods 
do not allow us to test this experimentally. Only by extensive 
and long-continued experiments more light will be thrown on 
this subject. The example which was mentioned of the seventy- 
years-old sea anemone, which reproduced itself successfully 
proves that the material for similar experiments is not deficient. 
In the vegetable kingdom forms will certainly be hit upon which 
will fully reward the difficulties of the experiment. 
Once a new species, modified and generally higher-differ- 
entiated, having arisen out of the first-born by gradual accumu- 
lation of the small deviations, intercrossing and _bastardising 
with the last-born descendants of the parent form, becomes rarer, 
copulation taking place by preference with specimens of the 
same species, and only exceptionally with representatives of the 
species which has lazged behind in its development. For this 
new species the same process sets in; here, too, the first 
born progeniture will surpass in the course of years the 
last-born, and will in its turn give rise to new modifications. 
And so ad infinitum. 
We now come t» another important point, which is in direct 
connection with the question, which are the last-, which the 
first-born. With most lower animals—Protozoa, Ccelenterata, 
Echinoderms, Worms—reproduction by fission is very common 
by the side of reproduction. Cut arms of starfishes grow to be 
complete starfishes after having passed the so-called ‘‘ comet” 
stage ; certain annelids divide themselves after one of the pos- 
terior body-segments have become converted into a head ; certain 
Nemertines break themselves into pieces under spasmodic con- 
tractions, each fragment being able to reproduce both head and 
tail ; Amcebz divide themselves into halves. 
Now it cannot be admitted that in fissiparous reproduction, 
heredity can come into play in the same measure as it can in the 
case of sexual reproduction. It is not even possible to deter- 
mine which of the two halves represents the older generation. 
Weissmann bas lately humorously said : if we fancy an Amoeba 
gifted with consciousness, she will think upon dividing into two, 
“*T now bring forth a child,” and there is no doubt that each 
half would look upon the other as the child, and upon itself as 
the mother. Weissmann has thus introduced the idea of the 
(approximate) immortality of the Protozoa, an idea which can 
also be adduced in favour of the hypothesis here maintained, 
and which at all events deserves to be mentioned by the side of 
the hypothesis proclaimed by Heckel and others, viz. that the 
Monera living in the present day are in no genetical connection 
with older ancestors from earlier periods, but have come into 
existence by the aid of repeated spontaneous generation.+ 
The same views hold good for the self-division of worms and 
Ccelenterata. Here too both parts are the direct continuation 
of a single individual which, although dividing, does not cease 
to exist. Coral reefs which principally multiply by division 
may be looked upon in the same way. 
Never, in case of fissiparous reproduction, does that myste- 
rious potentiation take place which brings together in the egg- 
cell and in the spermatazoon, not only the characteristic properties 
of father and mother, but of whole series of ancestors ; never 
in this case can the special process of fixation of a part of these 
latent forces, the process which we term heredity, take place to 
its full extent. Never can selection during embryonic and larval 
life, which, according to recent researches, plays a much more 
conspicuous part than was origina!ly expected, favour the stabi- 
lity of a variation, and thus lead to modification of the species, 
where multiplication by division takes place. 
In his chapter on pangenesis (‘‘ Origin of Species,” second 
edition, pp. 353 and 390) Darwin too touches upon this subject, 
and insists upon the fact that organisms produced asexually, 
consequently not passing through the earlier phases of develop- 
ment, ‘‘ will therefore not be exposed at that period of life when 
structure is most readily modified to the various causes inducing 
variability in the same manner as are embryos and young larval 
forms.” 
The series of generations which owe their origin to a-sexual 
and not to sexual reproduction, are thus in a much lesser degree 
liable to vary.2 And yet a variation of some sort must always 
first occur, in order that natural selection, acting upon it, may 
finally produce a definite modification of the species. Never- 
theless, fissiparous multiplication continues to play—and has 
always played—a very important part in the invertebrate king- 
dom, 4y the side of sexual reproduction. Thus the presumption 
is allowed, that where in the course of centuries a-sexual repro- 
duction has beea more predominant than sexual reproduction, 
a stagnation in development has re-ulted, the differentiation 
of those series of individuals and genera which have originated 
through sexual reproducti.n, in the meantime always continuing 
its regular course onwards. 
Both factors—the retardation of development by a-sexual re- 
production, and the acceleration of the development of the always 
first-born, make it very probable, in my opinion, that we have 
to look upon the more highly-developed groups of animals, and 
amongst these upon their higher-differentiated representatives, as 
forms which are separated from the original parent stock by a 
maximal number of aacestors, the number of times that a-sexual 
1 Lamarck had already, by this same assumption, attempted to overcome 
the difficulty. AN 4 A 
2 Observation tends to confirm this in a general way (vide Darwin, Z.c., 
DP. 353+ 
