SEPTEMBER 21, 1899] 
NATURE 
509 
individuals, but only in the production of a new individuality, is 
quite apparent, for in them two of the ordinary individuals of the 
species fuse to form one (many Protozoa). 
So that sexual reproduction gives us a new individuality which 
can spread to almost any extent by asexual reproduction. This 
asexual reproduction gives us a group of organisms which is 
quite different from a group cf organisms produced by sexual re- 
production. Whereas the latter groups constitute what we call 
species, the former group has, so far as I know, no special 
name, unless it be variety ; but variety is not a satisfactory name, 
for it has been used in another sense by systematisers. 
Heredity, then, is really applicable only to the appearance in 
a zygote of some of the properties of the gametes. A zygote has 
this property of one of the precedent gametes, and that property 
of the other, in virtue of the operation of what we call heredity ; 
it has a third property possessed by neither of the precedent 
gametes in virtue of the action of variation, the nature of which 
we have already examined. It is impossible to say which 
property of a gamete will be inherited, and it is impossible to 
predict what odd property will result from the combination of 
the properties of the two gametes. Of one thing only are we 
certain, that they are never the same in zygotes formed by 
gametes produced in immediate succession from the same 
parent. 
We may thus regard the activities of the zygote as the re- 
sultant of the dashing together of the activities of the gametes. 
Conjugation, then, is a process of the utmost -importance in 
biology ; it provides the mechanism by which organisms are 
able to vary, independently of the conditions in which they live. 
It lies, therefore, at the very root of the evolution problem; 
the power of combining to form a zygote is one of the funda- 
mental properties of living matter. 
Species. 
Now let us consider one of the effects of this property upon 
organisms. The effect to which I refer is the division of 
animals into groups called species. Species are groups of 
organisms the gametes of which are able to conjugate and pro- 
duce normal zygotes. Now in nature there appear to be many 
causes which prevent gametes from conjugating. First and 
most important of all is some physical incompatibility of the 
living matter which prevents that harmonious blending of the 
two gametes which is essential for the formation of a normal 
zygote. Very little is known as to the real nature of this in- 
compatibility ; in fact, it is hardly an exaggeration to say that 
nothing is known. It may be that there is actual repulsion 
between the gametes, or it may be, in some cases, at least, that 
the gametes are able to fuse, but not to undergo that intimate 
blending which is necesary for the production of a perfect 
zygote. In some cases we know that something like this 
happens ; for instance, a blend can be obtained between the 
horse and the ass, but it is not a perfect blend, the product or 
zygote being imperfect in one most important particular— 
namely, reproductive power. 
A second cause which prevents conjugation is a purely me- 
chanical one—viz. some obstacle which prevents the two 
gametes from coming together. As an instance of this I may 
refer to those cases amongst plants in which conjugation is im- 
possible, because the pollen tube is not long enough to reach 
the ovule. In yet other cases conjugation is impossible because 
the organisms are isolated from one another either geographic- 
ally or in consequence of their habits. There are probably 
many causes which prevent conjugation, but, whatever they 
may be, the effect of them is to break up organisms into specific 
groups, the gametes of which do normally conjugate with one 
another. 
In many cases, no doubt, the gametes of organisms which are 
kept apart in nature by mechanical barriers will conjugate fully 
if brought together. But in the great majority of cases it is 
probable no amount of proximity will bring about complete con- 
jugation. There is physical incompatibility. Here is a fruitful 
opening for investigation. Observations are urgently needed as 
to the real nature of this incompatibility. 
Importance of the Study of Variation, 
Another and most important effect o: conjugation is, as we 
have seen, the much-spoken-of constitutional or genetic varia- 
tions. They are, as we have already insisted, of the utmost 
importance to the evolutionist. Evolution would have been 
NO. 1560, VOL. 60] 
impossible without them, for it is made up of their summation. 
It becomes, therefore, desirable to find out to what extent a 
species is capable of varying. This can only be done, as Mr. 
Bateson has pointed out, by recording all variations found. Mr. 
Bateson, in his work already referred to, has carried this out, and 
has shown the way to a collection of these most important data. 
In order to carry it further, I would suggest that the collection 
be made, not only for structure, but also for function. This has 
been done largely for the nervous functions by psychologists and 
naturalists who pay special attention to the instincts of animals ; 
but we want a similar collection for other functions. For in- 
stance, the variations in the phenomena of heat and menstruation, 
and of rut amongst mammals, and so on. To do this is really 
only to apply the methods of comparative anatomy and com- 
parative physiology to the members of a species, as they have 
already been applied to the different species and larger groups 
of the animal kingdom. Such investigations cannot fail 
to be of the greatest interest. Indeed, when we have 
learnt the normal habits and_ structure of a species, 
what more interesting study can there be than the 
study of the possibilities of variation contained within it ? 
Then, when we know the limits of variability of any given specific 
group, we proceed to try if we can by selective breeding or 
alteration of the conditions of life alter the variability, and per- 
haps call into existence a kind of variation quite different in 
character from that previously obtained as characteristic of the 
species. 
The Evolution o Heredity and the Origin of Variation. 
These remarks bring me to the consideration of a point to 
which I am anxious to call your attention, and which is an 
important aspect of our subject. Has the variability of organisms 
ever been different from what it is now? Has or has not 
evolution had its influence upon the property of organisms as it 
is supposed to have had upon their other properties? There is 
only one possible answer to this question. Undoubtedly the 
variability of organisms must have altered with the progress of 
evolution. It would be absurd to suppose that organisms have 
remained constant in this respect while they have undergone 
alteration in all their other properties. If the variability of 
organisms has altered, it becomes necessary to inquire in what 
direction has it altered? Has the alteration been one of dimin- 
ution, or has it been one of increase? Of course, it is possible 
that there has been no general alteration in extent with the 
course of evolution, and that the alteration, on the whole, has 
been one of quality only. But passing over this third possi- 
bility, let us consider for the moment which of the two first- 
named alternatives is likely to have occurred. 
According to the Darwinian theory of evolution, one of the 
most important factors in determining the modification of or- 
ganisms has been natural selection. Selection acts by pre- 
serving certain favourable variations, and allowing others less 
favourable to be killed off in the struggle for existence. It 
thus will come about that certain variations will be gradually 
eliminated. Meanwhile the variations of the selected organisms 
will themselves be submitted to selection, and certain of 
these will be in their turn eliminated. In this way a group of 
organisms becomes more and more closely adapted to its sur- 
roundings ; and unless new variations make their appearance as 
the old unfavourable ones are eliminated, the variability of the 
species will diminish as the result of selection. Is it likely that 
new variations will appear in the manner suggested ? To answer 
this question we must turn to the results obtained by human 
agency in the selective breeding of animals. The experience of 
breeders is that continued selection tends to produce a greater, 
and greater purity of stock, characterised by small variability, 
so that if the selective breeding is carried too far, variation 
almost entirely ceases, and there is little opportunity left for the 
exercise of the breeder’s art. When this condition has been 
arrived at, he is obliged, if he wants to produce any further 
modifications of his animals, to introduce new blood—z.e. to 
bring in an individual which has either been bred to a different 
standard, or one in which the variability has not been so com- 
pletely extinguished. ; . 
It would thus appear, and I think we are justified in hold- 
ing this view, at any rate provisionally, that the result of con- 
tinued selection will be to diminish the variability, of a species ; 
and if carried far enough, to produce a race with ‘so little 
variability, and so closely adapted to its surroundings, that the 
