GENERATION AND DEVELOPMENT 733 



plumage, combs, feathered or clean legs, and extra toes, as opposed 

 to the normal condition. In the horse it applies to coat colour, and, 

 as Robertson* has shown, to ' staying ' power, to the number of 

 lumbar vertebrae, to the shape of the nasal bones, and shape of the 

 ears in the race-horse, and in Shetland ponies to hock callosities. 

 The principles apply much further in the case of plants, dealing with 

 such features as height, characters of flowering-head, leaves, and 

 stem, and have already been turned to useful economic account in 

 husbandry by the production of new and improved species. The 

 characters spoken of above are described in Mendelian phraseology 

 as Unit Characters. 



The union of the ovum and sperm-cell and their subsequent 

 division, described at p. 703, enables the essential facts of Mendelism 

 to be visualised. In Mendelian terminology either generative cell 

 is spoken of as a Gamete, and the organism resulting from the union 

 of two gametes is called a Zygote. We have seen that each gamete 

 contains half the number of chromosomes found in the body cells, 

 so that when the zygote is formed it is a double structure consisting 

 of an equal contribution of chromosomes from each parental gamete. 

 The resulting plant or animal retains in its cells throughout life the 

 double structure imparted by the gametes. As was stated on 

 p. 702, the inheritance of character is believed to be connected with 

 the chromosomes. 



We have seen that the cells in the living organism are divided 

 into two groups — body or somatic, and reproductive or germ cells 

 (p. 698). The absence of blending referred to on p. 731 refers solely 

 to the generative cells. The soma, or body, is the envelope con- 

 taining the generative cells, and between one generation and another 

 there is no continuity in the cells of the soma. The link between 

 generations lies in the germ cells. It is generally conceded that 

 environmental conditions may modify the somatic cells of the indi- 

 vidual, but that they exert no appreciable influence on the genera- 

 tive cells. Modification of the somatic cells not being heritable, it 

 is to the influence of natural selection transmitted through the 

 generative cells that variations must be attributed. 



When a pair of zygotes breeds true for any given unit character, 

 their gametes must in respect of that character be identical, and the 

 resulting offspring of their union carries in its generative cells the 

 same unit character. When two zygotes are mated which breed 

 true for opposite unit characters, such as tallness and shortness in 

 the common pea, the gametes produced by each, for the particular 

 unit character of height, are not identical, but opposite. The 

 resulting offspring in the first generation are not, however, in the 

 matter of height, a mixture of the two zygotes ; they take after 

 one parent or the other, not after both, and whichever characteristic 

 they follow is the dominant one ; that which the body cells have not 

 followed is the recessive. 



But though the body, or somatic, cells of the offspring have 

 followed the characteristic of either one or the other parent, their 

 generative cells are a mixture of the unit characters of both. In 

 spite of being mixed, there is no blending ; the cells producing tall 

 do not fuse with cells producing short plants, and subsequently 

 result in those of medium size. The determinant or factor which 

 produces tallness and the determinant or factor producing short- 

 ness, though both present in the generative cells of the zygote, are 

 as clearly and sharply separated as if only one factor were present. 



* See footnote, p. 736. 



