APPENDIX B 361 



In man, a peculiar dark-coloured condition of the urine known 

 as alkaptonuria, is inherited as a Mendelian recessive character." 1 

 To his examples may be added albinism, polydactylism, deaf- 

 mutism, colour-blindness, haemophilia, cataract, eye-colour, per- 

 haps colour and texture of hair, and a few other characters. 

 But the claim that these "suffice to show that Mendelian or 

 alternative inheritance is neither a rare nor an exceptional 

 phenomenon," is, to say the least, hardly justified. Even in cross- 

 breeding blended inheritance is extremely common. But of this 

 more anon. 



The Mendelian theory of he^dity is linked in the minds of 

 many of its supporters with the " mutation " theory of evolution. 

 In the present work we have maintained that, since the individual 

 in his own development recapitulates (with variations) the 

 development of his parent, therefore, of logical necessity, he must 

 recapitulate (with omissions and additions) the life-history of his 

 race. It follows that a progressive variation implies a complete 

 recapitulation of the life-history as presented by the parent, plus 

 a prolongation ; whereas a regressive variation implies an abbre- 

 viation of the life- history, and therefore a reversion to the ances- 

 tral type. We noted that, though variations occur all round the 

 specific mean, yet the more or less rapid disappearance of all 

 structures that are not maintained by selection, demonstrates that 

 the tendency to regression is stronger than the tendency to pro- 

 gression. Moreover, since progressive variations tend to disappear 

 in the next generation, since newly-evolved structures can only 

 be maintained by stringent selection, and since old-established 

 structures such as the vestigial remains of organs long useless are, 

 speaking comparatively, extremely stable, we concluded that time 

 was the principal element in the establishment of stability, and 

 that stability was never absolute, but always relative. But 

 the mutation theory accounts for stability differently. It sup- 

 poses that variations are of two distinct kinds an unstable kind, 

 which tends to disappear in offspring and which plays no part in 

 evolution, and a stable kind, which has no tendency to disappear, 

 which "selection alone can eliminate," 2 and on which, therefore, 

 evolution is founded. The unstable type of variation is termed 

 continuous or " fluctuating." The stable type is termed a muta- 

 tion, or a discontinuous variation. 



One of the main difficulties in the way of discussing the muta- 

 tion theory arises from the apparent divergence of opinion among 

 its supporters as to what constitutes a mutation. The word 

 " discontinuous " implies that it is a large variation, an " abnor- 

 mality," and it seems to be so understood by most of the supporters 

 of the theory. Thus Professor Castle writes : " We find within a 

 species two varieties, one larger than the other. Have they 



1 Popular Science Monthly, July 1905, pp. 197-9. 



2 Mendelism y by R. C. Punnett, Macmillan & Bowes, Cambridge, p. 52. 



