EVOLUTION 1099 



like ultra-microscopic billiard-balls. We dare say little more than 

 this — as yet, that they are protoplasmic differentiations causally 

 related to particular characters into which they develop in the 

 offspring arising from the fertilised egg-cell. But one gene may affect 

 several characters of the offspring, and one character of the offspring 

 is often the outcome of the co-operation of several genes. 



18. The largest fact as regards heredity is "the continuity of the 

 germ-plasm". That is to say, when a fertilised egg-cell is developing 

 into an offspring, some of the germinal material is kept apart in 

 completeness and generality, not sharing in body-making or differ- 

 entiation, but retaining the complete hereditary equipment of the 

 fertilised egg-cell. Such, more or less clearly, segregated germ-cells 

 form the reproductive organs and their ceUs, so that in the beginning 

 of the next generation there is precisely similar material to start 

 with. There would be complete hereditary resemblance, if it were 

 not for the causes, already considered, which evoke variations. 

 When body-cells or somatic cells retain, in spite of their division of 

 labour, a complete and uninhibited equipment of genes, there are the 

 well-known possibilities of asexual multiplication and regeneration. 



19. Development is the individual realisation of the inheritance. 

 The implicit becomes explicit, the latent patent, the invisible 

 visible. Out of the apparent simplicity of the fertilised egg-cell (in 

 all ordinary cases) there is developed the obvious complexity of the 

 organism. But normal development always implies an appropriate 

 "nurture" (e.g. food, oxygen, moisture, space, exercise); and a 

 developed character is a function — a product — of the inherited 

 "nature" and the environing "nurture". 



20. Experiment shows that the egg-cell and the sperm-cell 



contain a complete equipment of the specific characters; and they 



contain the same number of chromosomes, which is half the number 



(n\ 



( - 1 which is characteristic of the species in question (n) . The halving 



of the number is effected during the maturation of the germ-cells, 

 prior to fertilisation. But as regards the genes of novel characters 

 or individual characters, not part of the normal equipment of the 

 species, they may be carried in the egg-cell, but not in the sperm- 

 cell, or vice versa, or by both, or by neither. Thus when albinism, 

 or the thorough absence of pigment, occurs in an offspring, it means 

 that the genes for pigmentation have dropped out, in the course of 

 pre-fertilisation changes, from the particular ovum and from the 

 particular spermatozoon, that unite. 



21. When two members of a species pair, the resulting offspring 

 may show: 



[a) a replica of the two parents, if they were practically indis- 

 tinguishable — complete hereditary resemblance ; 



