NATURAL SELECTION 265 



of the evidence from one species and part of it from another. 

 There are few, if any, pairs of model and mimic (cf. footnote, 

 p. 260) in which all the necessary evidence is available for that 

 particular pair. In our view, therefore, while it is probable 

 that selection has played some part in establishing mimetic 

 resemblances, it is impossible as yet to estimate how large a 

 part, and certainly dangerous to use the mimicry theory as one 

 of the main lines of support to the Natural Selection theory. 

 When additional facts of the right kind have accumulated it 

 may be possible to come to a more definite conclusion. 



A secondary point, of some importance to the more general 

 questions with which we are dealing, concerns the relation 

 between colour-pattern and specific characters in mimetic 

 forms. We have presented some evidence that the patterns of 

 species involved in mimetic associations are often so polymorphic 

 that it is the finer details only, and not the broad lines of the 

 pattern, which must be regarded as specific. This question 

 requires examination on a quantitative basis, but it is probable 

 that, if the majority of cases of mimetic resemblance were 

 proved to be the result of Natural Selection, we would also 

 have to accept the view that specific differences in colour might 

 frequently have evolved under the same influence. 



(b) Less intensively studied cases. — (1) Adaptation to 

 life in torrents. — The study of the adaptation of animals of 

 various groups which live in torrents has been recently 

 developed by numerous workers. The study of aquatic insects 

 in particular has been pursued, particularly by Hubault, 

 Rousseau, Dodds and Hisaw and others. Annandale and 

 Hora started a special study of the fauna of Indian hill 

 streams, and Hora has recently (1930) published a masterly 

 summary of the general question. It will be easily understood 

 that this subject is part of the larger problem of the adaptation 

 of aquatic animals to various habitats. This particular aspect 

 has, however, received special attention. The following con- 

 clusions seem to be established : 



(i) The habits of species of the same genus claimed to 

 show adaptations to different speeds of water, etc., are too often 

 only summarily expressed, and there is a dearth of statistical 

 information — e.g. as to how regularly the members of a given 

 species are found in a given habitat. 



