202 Mr. W. Bateson. Heredity, Differentiation, 



individuals he would have seen this also. For in cases of confraternities 

 he must be familiar with the phenomenon of similar variations occurring 

 simultaneously in separate groups of differentiated members. 



But let us now suppose we could define differentiation from variation 

 in general, say, as orderly variation. Even so we could not distinguish 

 it unless its order were conspicuous. In a former paper,* Professor 

 Pearson wrote that " the very nature of the distribution of variation, 

 whether healthy or morbid, seems to indicate that we are dealing with 

 the sphere of indefinitely numerous small causes, which in so many 

 other instances has shown itself only amenable to the calculus of chance, 

 and not to any analysis of the individual instance." As I have on many 

 occasions stated, such a description accords ill with the observed facts 

 (A variation. Illustrations to the contrary are numerous and are 

 now becoming familiar ; and even in Professor Pearson's later works 

 references to them are not wanting. 



Does not, then, the presence of orderly differentiation, in various 

 degrees, compel us to an analysis of individual instances 1 In plain 

 language, we shall have to pick and choose our cases, and the value 

 of our coefficient of homotyposis will depend entirely on how we do 

 it. Has not Professor Pearson himself been so compelled in more than 

 one of his examples, notably in that of Nigella ? Has he any certainty 

 that such an analysis oiight not to have been made in other examples 

 also 1 



He speaks of the extreme difficiilty of determining whether his 

 material is homogeneous in respect of environment, but I miss from his 

 work any deep appreciation of the subtle and evasive quality of 

 differentiation. If any one would obtain a conception of this difficulty 

 let him go to any tree or large plant and set about pruning it, or better, 

 let him try to choose shoots for propagation. Until he tries, it seems 

 simple enough ; but when he begins he finds the shoots are of many 

 complexly differing kinds, and unless he has experience of pruning or 

 of propagation, he will not know which to choose. If he studies the tree 

 attentively, he will soon see that the kinds of shoots are largely definite 

 and, in fact, differentiated. The differentiation may be irregular or 

 regular. That of the leaves may or may not be correlated with that 

 of the shoots. The differentiations may be correlated with the age of 

 the wood, with the absolute size of the tree, they may be peculiar to 

 the variety, or they may be individual to the specimen and defy 

 analysis.! 



I am of course aware that Professor Pearson knows all this, but I 



* 'Phil. Trans.,' A, 1896, vol. 187, p. 255. 



t There are examples not only of differentiations occurring irregularly in one 

 species and regularly in another, but also of the separation of these very forms of 

 differentiation as characteristics of distinct varieties. See for instance the hetero- 

 phyllous Junipers and Cypresses. 



