OTHER THEORIES OF EVOLUTION 327 



that they indicate a determinate evolutionary tendency in 

 which it is alleged that no adaptive influence is at work. They 

 are : (1) Normal evolutionary series which appear to have been 

 uninfluenced by selection ; (2) Recapitulatory series in which 

 ' senescence ' is involved ; and (3) Excessive or over-complex 

 growth. (2) includes also certain forms of gigantism and com- 

 plexity of parts considered (e.g. by Beecher) to be produced as 

 a result of senescence. 



We will now review these three classes and the various 

 theories which have been put forward to explain them. We 

 ought to point out first that it is impossible to make a 

 hard and fast distinction between the three classes. Normal 

 evolutionary series are, no doubt, easy to distinguish from 

 extreme cases of progressive gigantism and over-elaboration 

 of ornamentation. But these types grade into one another. 

 Secondly, we should bear in mind that the various theories 

 we are to discuss may be competent to explain one or more 

 types of phenomena. Thus the theory of Fisher and Haldane 

 on the effect of selection on metrical characters determined 

 by many genes may be used to explain both normal ortho- 

 genetic processes and also excessive size. 



(1) The mere appearance of direction in an evolutionary 

 series and the assumption that it is non-adaptive cannot 

 weigh much as proof. Some of the evidence brought 

 forward to illustrate (1) is of this kind (e.g. Hogben, 1919 ; 

 Lull, 1 91 7), and is concerned with modifications that are 

 suspected of being non-adaptive but not proved to be so. 

 Instances of apparently meaningless histories of progressive 

 modifications could be multiplied almost indefinitely, and 

 certainly in the history of the Ammonites we find changes 

 (e.g. in the suture-lines and type of coiling) of such kinds that 

 it is very likely that they are not due to the direct effects of 

 selection. This conclusion is reinforced when we learn that 

 they are unaccompanied by any change in the contemporary 

 environment (Spath, in litt.). 



Haldane (1932, p. 194) has attempted to supply an explana- 

 tion of ' useless ' orthogenesis of this kind by reference to selec- 

 tion. He takes as his starting-points the effects of selection on 

 a metrical character determined by many genes, and Fisher's 

 analysis of the result of selection in favour of, e.g., larger size. 

 As far as we can understand the rather condensed argument, 



