328 THE VARIATION OF ANIMALS IN NATURE 



selection on any character represented by numerous genes 

 has the effect of increasing the number of advantageous genes 

 in such a way that they go on increasing after selection is 

 abandoned. ' The stature (e.g.) will thus, so to speak, over- 

 shoot the mark aimed at by selection. . . . We have here for 

 the first time an explanation on strictly Darwinian lines of 

 useless orthogenesis.' This is an ad hoc hypothesis and we do 

 not know if its premises have any foundation in fact (cf. 

 Chapter VII). 



As regards Eimer's own attempt to account for normal 

 orthogenesis of the Papilio type, it depends, of course, on two 

 assumptions, viz. (a) the limitation of the capacity for variation, 

 and (b) ' environmental pressure.' There is little doubt that 

 Eimer held that induced variation was inherited. As, how- 

 ever, he did not actually distinguish between the action of 

 the environment as eliciting a definite germinal change as 

 opposed to merely directing a predetermined heritable capacity, 

 his theory is scarcely relevant in the light of modern know- 

 ledge. 



(2) Recapitulatory series. — Palaeontologists have long been 

 familiar with sequences of fossil forms in which species and 

 larger groups seem to go through the same kind of develop- 

 mental changes as those which occur in the individual life- 

 time. Whether ontogeny recapitulates phylogeny or whether 

 phylogeny is an expanded version of ontogeny cannot be 

 discussed here. What we are concerned with is the undoubted 

 fact which is stated above, and we have to seek an explanation 

 for it. 



(a) A special feature of the recapitulatory process is that 

 when the life-cycles of related forms and the racial cycles of 

 related groups are studied, it is found that they do not always 

 follow the same programme. Tachygenesis and cenogenesis 

 (acceleration and retardation) intervene and modify the time 

 at which a structure or character appears in various groups. 

 Haldane (1932 and 1932a, p. 20) has claimed that ' the 

 gradual acceleration or retardation of a number of genes will 

 lead to orthogenetic evolution.' He shows that genes can 

 be classified according to the time at which they act. Some 

 act in the gamete stage (G), others in the maternal zygote 

 (MZ), others, again, on embryonic or immature structures 

 at various stages (Z1-Z3). Noting that there has been a 



