BI-PARENTAL REPRODUCTION 69 



at ally it is a case of regressive, not of progressive, variations. 

 No doubt progressive variations occur in abundance under 

 bi-parental reproduction, a tall dark pair may even have 

 children taller and darker than themselves ; but the point I 

 wish to emphasize is this that there is not an iota of evidence 

 that bi-parental reproduction is connected with progressive varia- 

 tions as cause and effect. On the contrary, we have every rea- 

 son to believe, on the authority of recent exact measurements, 1 

 that progressive variations occur quite as commonly among 

 parthenogenetic species as among forms sexually reproduced. 



115. It cannot, indeed, be too clearly recognized that no 

 form of bi-parental inheritance exclusive, particulate, or 

 blended is an example of progressive variation. 2 In exclusive 

 inheritance the peculiarities of one parent are rendered latent, 

 while the peculiarities of the other are not increased. In 

 particulate and blended inheritance the peculiarities of both 

 parents are to some extent lost or rendered latent. No 

 doubt, occasionally, a blended individual is better adapted to 

 his surroundings than either parent, or than the ancestral type. 

 Thus an animal with blended colours may be more in 

 harmony with the environment than his very dark father or 

 his very light mother ; thus, also, by intercrossing, man has 

 obtained several breeds of domestic animals useful to him 

 (e. g. sheep). But in all these cases the better adaptation to 

 the environment is not due to progressive variations. It is 

 due to regressive variations. 



116. Leaving common experience, we may turn to the 

 scientific evidence. This is even more decisive, inasmuch as 

 it is more striking ; for now, as a rule, we see reversion, not 

 to the immediate ancestry, but to the remote ancestral form. 

 If we cross varieties or species the general tendency is ever 

 towards great reversion. All that distinguishes the one 

 variety or species from the other, all the special characters 



1 See 85. 



2 Professor Cossar Ewart insists (Address to Zoological Section, 

 British Association, 1901) that the relative maturity of parents and 

 the degree of ripeness of the germ-cells are causes of variations. 

 Thus he found that, if very young pigeons and rabbits were 

 crossed with more mature animals, the latter were prepotent. He found 

 also, if a doe rabbit were mated some time before ovulation is due, that 

 the offspring resembled the sire ; but if she were mated thirty hours 

 after her proper pairing period, the offspring resembled the mother. 

 Evidently he believes that his experiments furnish evidence of the 

 causation of that kind of variability on which evolution progressive 

 evolution is founded But plainly there is no evidence of progressive 

 variation here. We have evidence only of some of the conditions which 

 determine prepotency. 



