504 
NATORE 
[SEPTEMBER 21, 1899 
is nothing more remarkable in a single bud on a tree departing 
from the usual character at a particular time of life, than in a 
particular hair ofa mammal doing the same thing. 
We have seen that, speaking broadly, genetic variation is 
connected with sexual reproduction, and it becomes necessary to 
examine this mode of reproduction a little more fully. What is 
the essence of sexual reproduction, and how does it differ from 
asexual ? What I am now going to say applies generally to the 
phenomenon whether it occurs in plants or animals. Sexual re- 
production is generally carried on by the co-operation of two 
distinct individuals—these are called the male and female 
respectively. They produce, by a process of unequal fission 
which takes place at a part of their body called the reproductive 
gland, a small living organism called the reproductive cell. The 
reproductive cell produced by the male is called in animals the 
spermatozoon, and is different in form from the corresponding 
cell produced by the female, and called in animals the ovum. 
The object with which these two organisms are produced is to 
fuse with one another and give rise to one resultant uninucleated 
organism or cell, which we may call the zygote. This process of 
fusion between the two kinds of reproductive cells, which are 
termed gavie’es, is called conjugation. The difference in struc- 
ture between the male and female gamete is a matter of secondary 
importance only, and isconnected with the primary function of 
coming into contact and fusing. The same may be said with 
regard to the so-called sexual differences of the parents of the 
two kinds of gametes, and to the powerful instincts which 
regulate their action. The conjugation of the male and female 
gamete, or the fertilisation of the ovum, as it is sometimes called, 
consists in the fusion of two distinct masses of protoplasm which 
are nearly always produced by different individuals. In the 
case of hermaphrodites, the term applied to organisms which 
produce both male and female gametes in the same individual, 
there is generally some arrangement which tends to prevent the 
male gamete from conjugating with the female gamete of the 
same parent ; but this phenomenon is not absolutely excluded, 
and takes place as a normal phenomenon in many plants and 
possibly in some animals. 
This fusion of the protoplasm of the two gametes gives us a 
uninucleated organism—for the fusion of the nuclei of the two 
gametes seems to be an essential part of the process—in which 
the potencies of the two gametes are blended. The zygote, as 
the mass formed of the fused gametes is called, is formed by the 
combination of two individualities, and is therefore essentially 
a new individuality, the characters of which will be different 
from the characters of both of the parents. This fact, which is 
not apparent in the zygote when first established, because the 
parts are hardly distinguishable by our senses, becomes obvious 
as soon as organs, with the appearance of which we are familiar, 
are formed. As a general rule this cannot be said to have 
occurred until what we call maturity has been nearly reached, 
because we are not familiar enough with the features of im- 
mature organisms to detect individual differences. But you 
may rest assured that such differences exist at all stages of 
growth from that of the uninucleated zygote till death. 
How the characters of the two parents will combine in the 
zygote it is impossible to predict, and the result is never 
the same even though the conjugations have been between 
gametes of identical origin. There may be an almost perfect 
mixture, the blending extending to even quite minute details ; 
or the characters of the one parent may predominate—be pre- 
potent, as we call it—over those of the other; or they may 
blend in such a way that the zygote offers characters found in 
neither parent. Or, finally, the features of one parent may 
come out at one stage of growth, those of the other at another 
stage. But, however the characters may blend, the product 
never exactly resembles the parents. The extent to which it 
differs from them is the measure of the variation. 
To resume, it will be observed that in the method of repro- 
duction sometimes called sexual two distinct processes occur. 
One of these is the real reproductive act, which consists in the 
production by fission of uninuclear individuals called gametes ; 
the second is the fusion of the gametes to form the zygote. The 
gametes are of two kinds, and the reason of there being two 
kinds is intelligible when we consider the parts they have to 
play. The male gamete is nearly always endowed with loco- 
motive power, and the female gamete is stored with food 
material to be used by the zygote in the first stages of growth. 
The destiny of these two uninucleated organisms is to fuse with 
one another, and so to give rise to a zygote which ultimately 
NO. 1560, VOL. 60} 
assumes the typical form of the species. As a general rule, the 
gametes have but a limited duration? of life unless they con- 
jJugate, and this is quite intelligible when we remember that 
they have no organs, e.g. digestive organs, suitable for the 
maintenance of life. It is rarely found that they have the power 
of assuming the form of their parent, unless they conjugate. 
This never happens in the case of the male gamete (at any rate 
in animals), and only rarely in that of the female. When it 
occurs—that is to say, when the ovum develops without con- 
jugation—we call the phenomenon parthenogenesis. 
genesis is found more commonly in Arthropods than in other 
groups, but it may be more common than is supposed.” 
In sexual reproduction then, in addition to the real reproduc- 
tive act, which is the division by fission of the parent into two 
unequal parts, the one of which continues to be called the 
parent, while the other is the gamete, there is the subsequent 
conjugation process. It is to this conjugation process that that 
important concomitant of sexual reproduction must be attributed, 
namely, genetic variation. We have thus traced genetic variation 
to its lair. We have seen that it is due to the formation of a 
new individuality by the fusion of two distinct individualities. 
We have also seen that in the higher animals it is always 
associated with the reproductive act. 
Let us now take a wider survey and endeavour to ascertain 
whether this most important process, a process upon which 
depends the improvement as well as the degradation of races, 
ever takes place independently of the reproductive act. In the 
Metazoa, to which for our present purpose I allude under the 
term higher animals, conjugation never takes place except in 
connection with reproduction. It is impossible from the nature 
of the process that it should do so, as I hope to explain later on. 
But among the Protozoa, the simplest of all animals, it is con- 
ceivable that conjugation might take place apart from reproduc- 
tion, and asa matter of fact itdoes doso. Let us now examinea 
case in which this occurs. Amongst the free-swimming ciliated 
Infusoria it frequently happens that two individuals become 
applied together, and that the protoplasm of their bodies becomes 
continuous. They remain in this condition of fusion for some 
days, retaining, however, their external form and not undergoing 
complete fusion. While the continuity lasts there is an exchange 
of living substance between the two bodies, in which exchange a 
bit of the nucleus of each participates. It thus happens that at 
the end of conjugation, when the two animals separate, they are 
both different from what they were at the commencement 
each has received protoplasm and a nucleus from its fellow, 
and the introduced nucleus fuses as we know with the 
nucleus which has not moved. It would therefore appear that 
all the essential features of the conjugation process, as we learnt 
them in the case of the conjugation of the gametes in the 
Metazoa, are present, and it is impossible to doubt that we 
have here an essentially similar phenomenon. The phenomenon 
differs, however, from the conjugation first described in this 
interesting and important respect, that the two animals separate 
and resume their ordinary life. It is true that their constitution 
must have been profoundly changed, but they retain their 
general form. I say that the constitution of the exconjugates, 
as we may call them after they are separated, must be different 
from what it was before conjugation, but so far as I know no 
difference in structure corresponding with this difference in con- 
stitution has been recorded. I feel no sort of doubt, however, 
that structural differences, z.e. variations, will be detected when 
the exconjugates are closely scrutinised and compared with the, 
animals before conjugation, and I would suggest that definite 
observations be made with a view to testing the point. Here, 
then, we have a case of conjugation entirely dissociated from 
reproduction, Other cases of a similar character are known 
among the Protozoa, though as a general rule the fusion between 
the conjugating organisms is complete and permanent. Among 
plants conjugation is generally associated with reproduction, but 
not always, tor in certain fungi? fusion of hyphze and consequent 
intermingling of protoplasm occurs, and is not followed by any 
1 Under favourable conditions, they may live a considerable time—e.g. the 
spermatozoon of certain ants, which are stated by Sir John Lubbock to live 
in some cases for seven years. 
2 It may be mentioned as a curious fact that parthenogenesis is rarely 
found in the higher plants, and, as I have said, is not known for the male 
gamete among animals. 
3 It must be mentioned, however, that in the case of these fungi the 
fusion of nuclei has not been observed, nor has it been noticed whether the 
habit, structure, or constitution of the conjugating plants are altered after 
the fusion. 
Partheno- — 
—a 
