Mode of Inheritance of Adaptive Characters. 



63 



is formed, which continues to branch and grow apically or 

 interstitially during the life of the plant, its cells giving rise 

 by growth and division to all the specialised tissues. Their 

 marked capability of regeneration shows that these meristem 

 cells retain, in greater degree than the specialised cells, the 

 power of reconstructing the whole plant. This power is also 

 found, in certain cases, in cells which have undergone some 

 degree of specialisation, and then seems to depend in part on 

 a sufficient number of cells being associated together. The 

 whole of these cells, which retain this power, must be looked 

 upon as a germinal area, the germinal powers of which fade 

 with imposed specialisation ; and there seems to be no hard 

 and fast line between the cells which constitute this area, 

 in its narrowest sense, such as has been called the track of 

 the germ cells, and others which have undergone some small 

 amount of specialisation. 



There is now reason to believe, from the studies of 

 botanists, that, in the formation of spores by tetrads, a 

 halving of the number of chromosomes occurs in the nuclei 

 of the dividing cells, and that this reduced condition accom- 

 panies the life cycle of the plant until a union of gametes 

 once more restores the original number. During that part of 

 the life cycle in which this reduced condition of the chromatic 

 substance obtains, there is a very wide power of regeneration 

 in all cells, even though, as in the higher mosses, some 

 degree of specialisation is attained. The whole plant in this 

 part of the life cycle must constitute a part of this germinal 

 area, even though special gametes are set aside to perpetuate 

 the species. By union of the gametes, a doubling of the 

 chromosomes in the nucleus is brought about, and, at the 

 same time, what has been usually termed a double ancestry, 

 and this is retained until the halving process is again gone 

 through in spore formation. 



In the Metazoa, from fertilisation of the egg, a generation 

 is formed which in many ways resembles the meristem 

 growth of plants. In the lower forms, long-continued 

 asexual growth and budding is common, sterilised tissues, 

 formed by, and accompanying other cells, which can re- 

 construct the animal in its entirety. In some cases a new 



