OTHER THEORIES OF EVOLUTION 339 



heterogony (and with them we may couple the original theory 

 of Fisher and the racial senescence theory in so far as senes- 

 cence is sometimes assumed to involve excessive size of parts), 

 it will be seen that none is particularly convincing. Haldane's 

 theory is perhaps the most satisfactory as a formal structure, 

 though it labours under the difficulty of (a) having to make 

 certain assumptions — e.g. that size is a character frequently 

 acted on by selection — and (b) being applicable only to charac- 

 ters determined by many genes. We are therefore impelled 

 to consider the question whether the phenomena of excessive 

 growth are due to an ' independent ' internal impulse. This 

 notion is usually rejected on the score either that it is a mere 

 nominal device and explains nothing, or that a generalised 

 ' impulse ' might actually turn out to be the effect of one of 

 the other principles just discussed. 



The second of these objections can, of course, be easily met 

 on its own ground. Either the evolutionary principles we 

 have just discussed satisfy us or they do not. If they do not 

 and if there still remains the appearance of some directive 

 force determining the magnitude of parts or of the whole 

 organism, we have to examine the claim that this force is 

 inherent in the vital activity of the organism. The charge 

 that ' orthogenesis,' as a self-determining principle, is a name 

 by which we merely describe but do not account for certain 

 facts, has already been discussed (p. 325). 



We have three questions to ask ourselves — (i) is there any 

 ground for believing that such an internal impetus is actually 

 demonstrable ? (ii) if there is, can we account for the pro- 

 gressive amplification of its results until they become of 

 phyletic (as opposed to individual) status? and (iii) if (i) and 

 (ii) are answerable in the affirmative, has this phenomenon 

 anything to do with the main problem of evolution, or is it 

 only a peculiar and special case ? 



(a) There is one fact that must attract our attention in 

 reviewing this subject — viz. the frequent association of exces- 

 sive growth with sexual differentiation. This fact, which is 

 the basis of Champy's theory of ' sexuality and hormones,' at 

 once raises the question whether, if in special cases (sexual 

 differentiation) exaggerated size is produced by the excess of 

 a specific hormone, the same may not be true of all cases of 

 excessive growth. May not all instances of excessive growth 



