SPONTANEOUS VARIATIONS 47 



simony. " Neither more, nor more onerous causes are to be 

 assumed, than are necessary to account for the phenomena." 



80. We leave this very important subject for the present. 

 At this stage it cannot receive adequate treatment, for, 

 until we have considered disease, we shall not be in possession 

 of all our evidence. Moreover, there is testimony that a 

 change of environment does really result in an "epidemic" 

 of variations. It is assumed, very generally, that such 

 epidemics are due to the direct action of the environment on 

 the germ-plasm. The facts, however, admit of an explanation 

 more in accordance with probability, but with which, also, 

 we are not in a position to deal at present. 1 Enough has 

 been said, however, to justify the conclusion that, except 

 perhaps in very rare instances, no variations arise through 

 the direct and immediate action of the environment. 



81. Bi-parental reproduction the intermixture of some- 

 what dissimilar germ-plasms has been alleged as a cause of 

 " fortuitous " variations. Professor Weismann and his fol- 

 lowers formerly supposed that it was the sole cause of 

 variability in all organisms except the very lowest. More 

 recently the influence of sexual reproduction as a cause of 

 variability has been denied altogether. 2 The truth un- 

 doubtedly lies between these two opinions. Sexual repro- 

 duction is a cause of many variations, but it is not the cause 

 of all variations. 



(a) In blended inheritance the characters of the parents 

 appear to unite in such a manner that the offspring is more 

 or less intermediate between the two. Thus the child of a 

 white man and a black woman usually blends the colour and 

 many other peculiarities of his parents in himself. If he 

 resembles one parent more than the other, then the former 

 is said to be prepotent. Thus "mules sometimes resemble 

 donkeys, at other times they resemble ponies." 3 



(&) In exclusive inheritance, one parent appears to transmit 

 his or her characters to the exclusion of the other, and, there- 

 fore, is entirely prepotent. Thus when black or albino 

 quadrupeds mate with individuals of another colour, the off- 

 spring often follow, in colour at least, exclusively after one or 

 other parent. A blue pigeon is prepotent over one of any 



1 See 143 and 163. 



2 " Variability is not a product of bi-parental inheritance. . . What- 

 ever be the physiological function of sex in evolution, it is not the 

 production of greater variability." (Pearson's Grammar of Science, pp. 

 473-4 ; ed. 1900.) 



3 The Penicuik Experiments, p. 10. 



