2 66 THE VARIATION OF ANIMALS IN NATURE 



(ii) There is little doubt concerning the differential adapta- 

 tion of genera. 



(iii) There is enough evidence that species of the same genus 

 do sometimes differ markedly in structural features that are of 

 obvious use in different rates of water-flow. Thus Dodds and 

 Hisaw (1924) describe three species of Baetis which live in 

 different habitats and are obviously modified to an increasing 

 flow of current. Morgan (19 13) and Lestage (1925) show that 

 the nymphs of Ephemerella deficiens and tuberculata differ in the 

 structure of the femur and claw, and that it is possible to corre- 

 late these differences with differences of environment. Hora 

 {I.e. p. 237) finds that the modification of the adhesive ap- 

 paratus of the species of the fish Glyptosternum can be correlated 

 with water-flow. 



(iv) One cannot fail to observe repeatedly that habitat 

 and structural differences are manifested between groups 

 of species rather than between individual species of a 

 genus — e.g. Tonnoir (1924) in his account of the Tasmanian 

 Blepharoceridae cites differences between groups of species. 

 One finds that several related species often live in the same 

 habitat. 



(v) Although in some adaptations (suckers of tadpoles, 

 shape of insects' bodies) a Lamarckian explanation may sug- 

 gest itself, it will hardly afford a satisfactory explanation of the 

 origin of special hairs or spines in the armature of claws and 

 legs in insects. 



The general impression that this work conveys is not 

 particularly convincing as far as the selective nature of inter- 

 specific differences is concerned. There are, it is true, certain 

 instances that are highly suggestive of a selective origin, but 

 one would not say that they were proved up to the hilt. It is 

 not enough, as we have already suggested, to point to examples 

 of different species taken in different habitats and to discover 

 that they differ in appropriate modification. It must be shown 

 (a) whether they are always found in such habitats, and 

 ib) whether species not modified in this fashion are ever found 

 in the habitats in question. Probably the evidence does show a 

 general adaptive tendency ; but it scarcely amounts to proof 

 of the regular correlation of structural and habitudinal 

 differences between allied species. 



(2) The colour of cuckoo's eggs.- — This subject has been 



