NATURAL SELECTION 307 



On the selectionist view all the parts of an organ are supposed 

 to vary and only very minute variations would be likely to 

 improve one element without upsetting the general balance, 

 and it is the selection of such minute variants that is assumed. 

 It appears to us that there is a certain danger in assuming 

 that important evolutionary processes are due to a type of 

 variation which is probably never demonstrated. Fisher 

 (1930, pp. 14-16) has attempted to show that it is fallacious 

 to suppose that the advantage conferred by a variation only 

 very slightly in a favourable direction can be too small to 

 be of survival value (but cf. p. 223). When we are dealing 

 with a single organ or instinct the alteration of which in a 

 particular direction is clearly beneficial to the animal, then 

 Fisher's argument carries more weight. But it is rather 

 different with regard to complex organs, where it would appear 

 that the alteration of one part would be of no value without 

 the correlated variation in all the other parts. If, however, 

 we postulate such correlated variation, we are abrogating from 

 selection the most important part in the formation of complex 

 organs. We may consider as an example the eyes found in 

 Lamellibranch molluscs. The most specialised type is seen in 

 the Pectinidae, but in other families all gradations of structure 

 are found (Dakin, 1928). There appears to be very little 

 correlation between mode of life and eye-development. Some 

 actively swimming species have complex eyes, others have 

 simple eyes or none at all, and the same applies to the sedentary 

 species. Experiments on Pecten show that, in all probability, 

 even its very specialised eye does no more than perceive differ- 

 ences in light and shade, chemical stimuli being far more potent 

 than light in directing its movements. Thus we appear to 

 have an extremely complex organ of little adaptive value. 

 If such an organ can develop largely without the influence of 

 selection, then other eyes which are more obviously useful to 

 their possessor may also partly evolve without selection. The 

 problem is not one open to very convincing solution either 

 way and should be left sub judice. 



Fisher (1930, pp. 38-41) and especially Haldane (1932, 

 p. 174) have attempted to show that no organ can be too 

 complex for Natural Selection to evolve. The argument is a 

 mathematical one based on the assumption that every part 

 of an organ will be varying independently in all directions. 



