TRANSACTIONS OF SECTION D. 773 
can by selective breeding or alteration of the conditions of life alter the variability, 
and perhaps call into existence a kind of variation quite different in character from 
that previously obtained as characteristic of the species. 
The Evolution of 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 beeu one of 
diminution, 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 possibility, 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 organisms has been natural selection. 
Selection acts by preserving 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 surroundings; and unless new variations 
make their appearance as the old unfavourable ones are eliminated, the variability 
of the species will diminisu 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 completely extinguished. 
It would thus appear, and I think we are justified in holding this view, at any 
rate provisionally, that the result of continued 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 slightest alteration 
in the conditions of life will cause extinction. 
If selection tends to diminish the variability of a species, then it clearly follows 
that as selection has been by hypothesis the most important means of modifying 
organisms, variation must have been much greater in past times than it is now. 
! The expression ‘extinction of species’ seems to be used in two senses, which are 
generally confused. Firstly, a species may become modified so that the form with 
which we are familiar gradually gives place to one or more forms which have been 
gradually produced by its modification. That is to say, a character or series of 
characters becomes gradually modified or Jost in successive generations. ‘This is not 
really extinction, but development. Secondly, a species may gradually lose its 
variability, and become fixed in character. If the conditions then change, it is 
unable to adapt itself to them, and becomes truly extinct. In this case it leaves ng 
descendants, We haye to do with degth, and not with development, 
