12 PRESIDENT’S. ADDRESS. 
about us variations occurring of such a kind as to warrant faith in a 
contemporary progressive Evolution? ‘Till lately most of us would 
have said ‘ yes’ without misgiving. We should have pointed, as 
Darwin did, to the immense range of diversity seen in many wild 
species, so commonly that the difficulty is to define the types them- 
selves. Still more conclusive seemed the profusion of forms in the 
various domesticated animals and plants, most of them incapable of 
existing even for a generation in the wild state, and therefore fixed 
unquestionably by human selection. These, at least, for certain, are 
new forms, often distinct enough to pass for species, which has arisen 
by variation. But when analysis is applied to this mass of variation 
the matter wears a different aspect. Closely examined, what is the 
“variability ’ of wild species? What is the natural fact which is 
denoted by the statement that a given species exhibits much variation ? 
Generally one of two things: either that the individuals collected in one 
locality differ among themselves; or perhaps more often that samples 
from separate localities differ from each other. As direct evidence of 
variation it is clearly to the first of these phenomena that we must 
have recourse—the heterogeneity of a population breeding together in 
one area. This heterogeneity may be in any degree, ranging from 
slight differences that systematists would disregard, to a complex 
variability such as we find in some moths, where there is an abund- 
ance of varieties so distinct that many would be classified as specific 
forms but for the fact that all are freely breeding together. Naturalists 
formerly supposed that any of these varieties might be bred from any 
of the others. Just as the reader of novels is prepared to find that 
any kind of parents may have any kind of children in the course of the 
story, so was the evolutionist ready to believe that any pair of moths 
might produce any of the varieties included in the species. Genetic 
analysis has disposed of all these mistakes. We have no longer the 
smallest doubt that in all these examples the varieties stand in a regular 
descending order, and that they are simply terms in a series of com- 
binations of factors separately transmitted, of which each may be 
present or absent. t 
The appearance of contemporary variability proves to be an illusion. 
Variation from step to step in the series must occur either by the 
addition or by the loss of a factor. Now, of the origin of new forms 
‘ by loss there seems to me to be fairly clear evidence, but of the con- 
temporary acquisition of any new factor I see no satisfactory proof, 
though I admit there are rare examples which may be so interpreted. 
We are left with a picture of variation utterly different from that 
which we saw at first. Variation now stands out as a definite physio- 
logical event. We have done with the notion that Darwin came latterly 
to favour, that large differences can arise by accumulation of small 
