330 THE SIGNIFICANCE OP SEXUAL REPRODUCTION 



variation. An increase of variation is produced because the gem- 

 mules which reach the egg- through the sperm-cell may unite or con- 

 jugate 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 con- 

 cludes 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 1 . I 

 will refrain from considering here the opinion of several authors 

 (Darwin, Flourens, and Bechstein) that the influence of the ass is 

 stronger in both cases, only predominating to a less extent when 

 the ass is the female parent ; and I will for the sake of brevity 

 accept Brooks' opinion that in these cases the influence of the 

 father is greater than that of the mother. Were this so in all 

 cross-breeding between different species and in all cases of normal 

 fertilization, we should be compelled to conclude that the influ- 

 ences of the male and female germ -plasms upon the offspring 



1 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. I after- 

 wards saw two true hinnies at Professor Kuhn's Agricultural Institute at Halle. 

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

 tractable and gcod-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. 



