OTHER THEORIES OF EVOLUTION 341 



indicate the activity of some physiological momentum, we 

 have still to find some explanation of how changes of this order 

 become characteristic of whole populations. Granting that they 

 may arise in individuals, how do such individuals multiply ? 

 Dendy (I.e. p. 2) has suggested that in the first instance a 

 monstrous structure may have been useful, and the normal 

 inhibitions may have been subject to the adverse effects of 

 selection favouring individuals in which they were less well 

 developed. The inhibiting effect may have been thus pro- 

 gressively minimised until it was lost altogether, and the size 

 of the given organ ran riot until the lineage so affected was 

 extinguished by its excess. 



So far we seem to be in a logical impasse. It is asserted 

 that single mutations must have a certain adaptive advantage 

 if they are to spread and become a permanent character of 

 whole populations. Yet we seem to be dealing in all types 

 of Orthogenesis with populations exhibiting structures of 

 which the adaptive value, at least in the final stages of their 

 development, seems not only questionable but in the highest 

 degree improbable. Are there ways more effective than 

 those we have suggested (p. 318) in which a non-adaptive 

 character may spread, or are we wrong in rating (e.g.) the 

 growth of the canines in Babirusa and the Machaerodonts as 

 non-adaptive ? In questions of this kind explanations which 

 rely on the existence of a physiological momentum meet just 

 as many difficulties as do those which depend on Natural 

 Selection. 



Of a different order from the phenomena discussed above, 

 but similar to them in so far as they appear to be determined 

 by factors inherent in the organism itself, are the peculiar 

 manifestations of growth seen in patterns of various kinds 

 (e.g. in the coats of mammals, the colour and ornamentation 

 of Mollusc shells, the venation of insect wings, the spirals and 

 carination of shells, and so on). The evolution of such forms 

 has been referred to internal principles of growth ultimately 

 determined either by the material of the living substance or 

 by the differential growth-rate of the parts of the organism itself 

 (Bateson, D'Arcy Thompson). Although we admit that many 

 such patterns cannot be shown positively to have no adaptive 

 value, so many of them are like the patterns produced as the 

 result of non-vital activities that one can but suspect that they 



