Oct. 28, 1886 | 
individuals. But it is otherwise in ‘he case of the minute differ- 
ences which are characteristic of the individual, because every 
individual possesses them, and each in a different manner. Here 
a compensation of the differences could occur only if the entire 
species were composed of but few individuals, but the number of 
individuals which constitute a species is in most cases practically 
unlimited, and thus an intercrossing of all of these with all the 
others with the result of the compersation of the individual 
differences is impossible. 
The process of amphigonic reproduction is able to bring about 
what is absolutely essential to the working of selection as an all- 
powerful agent in development, namely, the summing up of the 
small individual differences it has to deal with in the direction of 
the result aimed at, and the production by this means of new 
characters. Such summing up of minute characters is impossible 
in the case of species with non-sexual reproduction. In the 
course of successive sexual generations the differences between 
the individuals of a species must continually increase, not as 
regards the greater differences, but as regards the constantly 
new combinations of individual peculiarities which are formed. 
Imagine a number of individuals of a species distinguished each 
from the others by a few hereditable peculiarities ; in the next 
generation no single individual could be like the others, they 
uuust all be different. Further, not one of the progeny could be 
identical with one of their parents, since each has combined 
in it the hereditable tendencies of two parents, its organism 
being as it were a compromise between the developmental ten- 
dencies of both. In the third generation the hereditary 
tendencies of two individuals of the second generation are 
combined. But since the germ-plasma is no longer simple, 
but is already compounded of two individual kinds of germ- 
plasma, an individual of the third generation will represent a 
compromise between four different hereditable tendencies. In 
the fourth generation eight such tendencies must be combined ; 
in the fifth, sixteen; in the sixth, thirty-two. Each of these 
tendencies may show its effect more or less marked in this or 
that part of the organism as it is developed, and thus in the 
sixth generation a number of the most different combinations of 
the individual peculiarities of the ancestors may be exhibited ; 
combinations such as never before existed in the history of the 
species, and such as never can occur again. 
The prepotency of the various kinds of idioplasma which com- 
pose the germ-plasma of the germ-cells of each individual 
must vary in intensity at various periods of its life. This must be 
assumed in order to account for the fact that the several children 
of the same parents are never exactly alike. ‘lhe presence of 
the small individual peculiarities postulated as displayed in the 
hypothetical series of individuals of the first generation is ac- 
counted for as having arisen, not amougst the higher organisms, 
the Metazoa, but amongst their unicellular Protozoan ancestors. 
Amongst these there is no distinction between body-cells and 
germ-cells ; their reproduction is by division, and in this case every 
modification of the body of the organism, every individual pecu- 
arity, however produced during the course of life, must be 
directly transmitted to the offspring. Here parent and child 
are in a certain sense still one and the same being. The child 
is a portion of the parent, and commonly half the parent. The 
conditions are entirely different from those oecurring in the 
sexual reproduction of the Metazoa, by which acquired pecu- 
liarities are not transmitted. 
Hereditarily transmissible variation having arisen in the Proto- 
zoa by the direct action of external causes, was retained by the 
Metazoa when they became developed; and as amphigonic 
sexual reproduction arose at the same time, the variation be- 
came thereby enhanced and increased in complication, and pre- 
served in ever-changing combinations. 
In the theory of sexual reproduction put forward by Prof. W. 
K. Brooks, of Baltimore, in 1883,! there is a resemblance to the 
views here maintained, in that sexual reproduction in it also is 
regarded as the means which is employed by Nature to produce 
variations, but the mode in which the influence is supposed to 
act is entirely different in the two theories, Brooks attributing 
the main effects in variation to the inheritance of acquired pecu- 
liarities, and to what may be termed the ‘‘circulation of the 
germ-plasma.” His theory is a modification of Darwin’s “‘ pan- 
genesis.” He assumes, like Darwin, that each cell of the body 
of higher organisms throws off minute gemmules, not always 
and under all conditions, but only when they encounter un- 
t W. K. Brooks, ‘‘The Law of Heredity: a Study of the Cause of 
Variation and the Origin of Living Organisms ’’ (Baltimore, 1883). 
NATURE 
631 
usual circumstances of existence. Whenever the normal func- 
tions of any component cell of the body are disturbed, ‘‘it 
throws off small particles which are the germs or gemmules 0 
this particular cell.” These gemmules are, according to Brooks, 
capable of passing to all parts of the body, and may pass into 
an oyarian ovum or a bud, but the male germ-cell has a special 
attractive force for them, gathering them within itself and storing 
them up. It is one of the peculiar merits of the theory of the 
continuity of the protoplasm that it dispenses with all necessity 
for any hypothesis of a circulation of gemmules or germ-plasma, 
so complicated and difficult to understand as these hypotheses 
are. As soon as the inheritance of acquired peculiarities is 
denied, such hypotheses are not required. Brooks further 
differs widely from Weismann in ascribing to the two germ-cells 
concerned in impregnation a different 7d/e in the process, the 
egg-cell or conservative principle being supposed to be charged 
with comparatively few gemmules, the sperm-cell, or radical 
principle, with many. This view, that the male germ-cell has 
a different part to play during the construction of the embryo 
from the female germ-cell is considered by Weismann to be un- 
tenable, because it is in contradiction with the simple matter of 
observation that human children on the whole are able to inherit 
just as many characteristics from the mother as from the father. 
Apparently, the only function of amphigonic reproduction is 
the production of a supply of hereditable individual peculiarities 
on which selection may work, and as the development of the 
whole higher organic world depends on these processes, the part 
which amphigony has to play in Nature is one of the most stu- 
pendous conceivable, and hence its wide, almost universal, distri- 
bution in theanimaland vegetable kingdoms. Nevertheless, itisnot 
in any way contended that amphigonic reproduction originally 
came into existence in order to render the production of species 
possible. The process must have been already present before 
it evoked the hereditable variability, and its first appearance 
must therefore have had another cause. This cannot at present 
be explained with any certainty, but the key to the problem lies 
in the conjugation of the Protozoa, the predecessor of amphi- 
gonic generation. The fusion of two unicellular individuals into 
one, the simplest form of conjugation, must have a direct and 
immediate action which is of advantage to the existence of the 
species concerned. The view taken by Hensen and E. Van 
Beneden and others, that conjugation, as well as sexual reproduc- 
tion generally, effects a rejuvenescence of the organism, is dis- 
cussed and rejected as unsatisfactory. ‘*‘ The entire conception 
of rejuvenescence has something indefinite and nebulous about 
it. The assumption of the necessity of a rejuvenescence of life, 
brilliant as it is, is scarcely to be reconciled with our other con- 
ceptions of the nature of life based on purely physical and mecha- 
nical forces. How can it be conceived that an infusorian which has 
at last by repeated division exhausted its power of reproduction 
can recover this power by fusion with another similarly effete infu- 
sorian? Twice nothing does not make one, and if it be assumed. 
that each infusorian retains half its reproductive force, the two 
combined would result only in one whole, but this could hardly 
be termed rejuvenescence. It amounts only to an addition 
such as, under other conditions, would be produced by simple 
growth.” 
Tt is best to assume in the present state of knowledge that 
living matter is endowed with a power of unlimited assimilation 
and consequently unlimited capability of reproduction, and that 
the form of reproduction, whether sexual or asexual, of itself 
exercises no influence on the duration of the process. It has not 
been proved that reproduction by division can never take place 
indefinitely. The phenomenon of parthenogenesis is strongly 
against the hypothesis of rejuvenescence, for, if. impregnation 
represents a rejuvenescence, and essentially consists in a com- 
bination of energies and materials which in virtue of their 
differences give rise to the development of reproductive force, it 
is difficult to understand how occasionally the same reproductive 
force can be produced by one of the two materials, only the egg- 
cell, The common assumption that in the case of partheno- 
genesis a single impregnation suffices for a whole series of gene- 
rations has no grounds of support, and is at variance with the 
fact that the same egg which may develop parthenogenetically 
(in the queen bee) is also capable of fertilisation. 
The primitive action and meaning of conjugation may at present 
be best provisionally defined as a strengthening of the forces of the 
organism in relation with reproduction, which occurs when, on 
account of external causes, such as atmosphere, temperature, and 
want of food, the growth of the single animal to the size necessary” 
