64 THE PRINCIPLES OF HEREDITY 



existence of dormant tendencies enables us to explain the 

 fact, otherwise inexplicable by the theory of recapitulation, 

 that individuals who revert to a remote ancestor may 

 reproduce traits that individuals who do not revert do not 

 recapitulate. When the white pigeon was evolved from the 

 blue pigeon a new chapter was added to the life-history, but 

 not in prolongation of it. To use a somewhat clumsy 

 illustration, it was pasted over the last leaves of the volume. 

 In this case therefore reversion means not a shortening of 

 the volume, but a stripping away of the superimposed pages. 



105. The frequency with which characters become 

 dormant demonstrates that latency is one of the means by 

 which species are adapted to their environments. Useless, 

 or rather injurious, characters are sometimes eliminated thus. 

 Generally speaking latency seems to occur only when evolu- 

 tion is rapid when some character, long and strongly estab- 

 lished, has become injurious and must be quickly replaced by 

 one more useful. Thus various wild characters in many 

 domesticated animals and plants have become latent under 

 stringent artificial selection. 



106. It is of course possible that characters may some- 

 times become dormant for reasons other than because they 

 are incompatible with newer and more useful characters 

 which replace them. Without being replaced by anything 

 they may be suppressed by Natural Selection merely because 

 they have become injurious to the species. 



107. A character, then, may disappear from a line of 

 individuals in two distinct ways. It may be quite eliminated 

 from the race by simple reversion to ancestors more and 

 more remote till at last, as regards this character, that 

 ancestor is approximated to one who existed at a time anterior 

 to its first appearance. In this case it will never appear again 

 except as a result of a fresh series of progressive variations. 

 Or it may become dormant, in which case it will reappear 

 only as a result of regressive variations. 



problems of heredity we must always bear in mind that the germ- 

 plasm, not the individual, is the real subject of discussion. The germ- 

 plasm undergoes gradual changes ; the individual enables us to ascertain 

 and indicate those changes. We have no reason to suppose that the 

 germ-plasm is divided into and is compounded of particles representing 

 separate individuals. On the contrary it is probable that there is 

 continuity of the germ-plasm, and therefore that the individual is 

 nothing more than an incident in its career, a dwelling-place where 

 shelter and nutrition are obtained. We judge of the nature of the germ- 

 plasm and of the changes it undergoes by the characters of the houses 

 which it builds and temporarily inhabits. 



