310 THE ZOOLOGIST. 



range of diversity seen in many wild species, so commonly that the 

 difficulty is to define the types themselves. 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 have 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. Natura- 

 lists 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 might 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 combinations of factors separately 

 transmitted, of which each may be present or absent. 



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 contemporary 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 physiological event. We have done with the 

 notion that Darwin came latterly to favour, that large differences can 

 arise by accumulation of small differences. Such small differences 

 are often mere ephemeral effects of conditions of life, and as such are 

 not transmissible ; but even small differences, when truly genetic, 

 are factorial like the larger ones, and there is not the slightest reason 

 for supposing that they are capable of summation. As to the origin 

 or source of these positive separable factors, we are without any 

 indication or surmise. By their effects we know them to be definite, 

 as definite, say, as the organisms which produce diseases ; but how 

 they arise and how they come to take part in the composition of the 



