FACTOKS OF INHEKITANCE. 59 



that it would eclipse even such triumphs as the practical 

 application of electricity, or the use of steam power. 



A word may here be said about the determination of 

 sex, although space will not allow us adequately to discuss 

 this most interesting and important problem in evolution. 

 What determines that a given individual shall be male, 

 female, or hermaphrodite? Since, in the vast majority 

 of cases, when the sexes are separate, they appear in 

 equal numbers, it might seem that here is an excellent 

 instance of mendelian segregation. We may suppose, as 

 Geoffrey Smith suggested, that one sex is a homozygote 

 and the other a heterozygote for sex-determining factors. 

 The heterozygote parent would then yield male-producing 

 and female-producing gametes in equal numbers, and the 

 sex of the offspring would b determined at fertilisation. 

 Working on quite different lines, M'Lung, Wilson, and 

 others have shown that in certain insects there are two 

 kinds of spermatozoa, one with the normal number of 

 chromosomes, and the other with one of these reduced in 

 size or absent. These observations have been extended 

 to other forms, and it is now established that in, at all 

 events, a large number of organisms, the egg develops 

 into a male or a female according as it is fertilised by a 

 male-producing or female-producing gamete. Sex, then, 

 would in these cases be determined from the first by the 

 presence of the necessary germinal factors. Yet there 

 are undoubted instances where the sex of an animal is 

 influenced by the environment. In the case of the spider 

 crab, Inachus, attacked by the parasitic crustacean 

 Sacculina,) not only does the male undergo "parasitic 

 castration," and acquire the external characters of the 

 female, as shown by Giard, but, as Geoffrey Smith proved, 

 on recovery from the attack the male crab develops ova 

 as well as spermatozoa. This seems to show that one 

 sex may really carry the factors for both, a conclusion 

 borne out by the well-known fact that female birds often 

 in old age acquire the secondary sexual characters of the 



