HEREDITY BATESON. 367 



evolutionary change. Do we then as a matter of fact find in the 

 world 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 b}^ 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 varia- 

 tion? Generally . one of two things; either that the individuals col- 

 lected 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 phe- 

 nomena tha^" T,o must have recourse — ^the heterogeneity of a popula- 

 tion breeding together in one area. This heterogeneity may be in any 

 degree, ranging from slight differences that systematists would disre- 

 gard, to a complex variability such as we find in some moths, where 

 there is an abundance 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 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 clgubt that in all these ex- 

 amples 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 illu- 

 sion. 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 in- 

 terpreted. 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 Dar- 

 win came latterly to favor, that large differences can arise by accumu- 



