v.] IN THE THEORY OE NATURAL SELECTION .339 



The transformation of species is therefore believed to take 

 place, for the most part, as follows : — those parts which are 

 placed in unfavourable conditions by the operation of external 

 influences, and which have varied, throw off gemmules which 

 reach the sperm-cells, and the latter by fertilization further 

 propagate the variation. An increase of variation is produced 

 because the gemmules which reach the ^g% through the 

 sperm-cell may unite or conjugate with parts of the former 

 which are not the exact equivalents of the cells from which the 

 gemmules arose, but only very nearly related to them. Brooks 

 calls this ' hybridization,' and he concludes that, just as hybrids 

 are more variable than pure species, so such hybridized cells 

 are also more variable than other cells. 



The author has attempted to work out the details of his 

 theory with great ingenuity, and as far as possible to support 

 his assumptions by facts. Moreover, it cannot be denied that 

 there are certain facts which seem to indicate that the male 

 germ-cell plays a different part from that taken by the female 

 germ-cell in the formation of a new organism. 



For example, it is well known that the offspring of a horse 

 and an ass is different when the male parent is a horse from 

 what it is when the male parent is an ass. A stallion and a 

 female ass produce a hinny which is more like a horse, while 

 a male ass and a mare produce a mule which is said to be more 

 like an ass ^ I will refrain from considering here the opinion 

 of several authors (Darwin, Flourens, and Bechstein) that the 



^ This seems to be the general opinion (see the quotation from Huxley 

 in Brooks' ' Heredity,' p. 127) ; but I rather doubt whether there is such 

 a constant difference between mules and hinnies. Furthermore, I 

 cannot accept the opinion that mules always resemble the ass more 

 than the horse. I have seen many mules which bore a much stronger 

 likeness to the latter. I believe that it is at present impossible to decide 

 whether there is a constant difference between mules and hinnies, 

 because the latter are very rarely seen, and because mules are extremely 

 variable. I attempted to decide the question last winter by a careful 

 study of the Italian mules, but I could not come across a single hinny. 

 These hybrids are very rarely produced, because it is believed that they 

 are extremely obstinate and bad-tempered. 1 afterwards saw two true 

 hinnies at Professor Kiihn's Agricultural Institute at Halle. These 

 hinnies by no means answered to the popular opinion, for they were 

 quite tractable and good-tempered. They looked rather more like 

 horses than asses, although they resembled the latter in size. In this 

 case it was quite certain that one parent was a stallion and the other 

 a female ass, — A. W. 1889. 



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