94 SECTIONAL ADDRESSES 



also on eye-size — when the eye is smaller, the melanin is more crowded 

 and the eye looks darker. Thus a mutation affecting the relative rate of 

 eye-growth alters the depth of eye-pigmentation. 



It would seem inevitable that many of the apparently useless features 

 used in diagnosing species are correlated characters of this type. Not 

 only this, but the development of such correlated characters during 

 evolution may simulate orthogenesis. One of the most convincing bits 

 of evidence for orthogenesis was the discovery of Osborn that horns of 

 the same type arose independently in four separate groups of Titanotheres. 

 The study of relative growth, however, has provided a simpler explanation. 

 The horns of Titanotheres are, like most horns, allometric, increasing 

 in relative size with the absolute size of the animal, and not appearing 

 at all below a certain absolute size. Given the potentiality of frontal 

 horns in the ancestral stock, their independent actualisation in the different 

 groups becomes inevitable so soon as a certain threshold of body-size is 

 reached. Increase of body-size is probably advantageous up to a limit ; 

 if so, the horns are the useless correlate of a useful character. It would 

 be more accurate to say initially useless, since presumably once they ap- 

 peared they were employed in fighting. That they later became useful 

 is rendered probable by the brilliant analysis of Hersh, who has shown 

 that after a certain period in their evolution the allometry of the horns 

 became intensified. 



Generally speaking, change in absolute size is almost certain to produce 

 numerous correlated changes in proportions, and change in relative size 

 of an organ is quite likely to be accompanied by correlated changes in 

 various characters. In addition, continued increase in absolute size will 

 so increase the relative size of an allometric organ that it will eventually 

 approach the boundary of disadvantage. Selection may then operate 

 to reduce its rate of growth, or, if conditions alter rapidly, the organism 

 may be caught napping in an evolutionary sense, and be extinguished. 

 This may apply to the antlers of the Irish elk and the fantastic horns 

 of some beetles. 



The claim that the concept of rate-genes is important would thus seem 

 to be justified. It has illuminated the evolutionary aspect of recapitu- 

 lation, neoteny, fcetalisation, clandestine evolution, and apparently useless 

 characters, as well as helping to a simpler understanding of the innumerable 

 cases of quantitative evolution. 



The Results of Selection, Good and Bad. 



Examples such as those of polytocous mammals, of abundant versus 

 rare species, and of allometric organs, show how the type and course of 

 evolution may be altered according to the type of organism or of biological 

 machinery on which it has to work. We may mention a few other cases 

 to illustrate this general principle. The most striking is, I think, that 

 of the social insects. Haldane has demonstrated that only in a society 

 which practises reproductive specialisation, so that most of the individuals 

 are neuters, can very pronounced altruistic instincts be evolved, of a type 

 which ' are valuable to society but shorten the lives of their individual 

 possessors.' Thus, unless we drastically alter the ordering of our own 



