68 THE BIOLOGICAL PROBLEM OF TO-DA Y 



logical classification ; in contrast, we may denote 

 the other characters as constitutional, or species, 

 characters. 



No doubt tissue cells are in the same case as 

 genital cells. So far as microscopical characters go, 

 egg cells and spermatozoa are wonderfully alike in 

 all the mammalia ; in many cases we could not dis- 

 tinguish between those of different animals. But, 

 because they bear the specific characters, we cannot 

 doubt but that they are as distinct as are the 

 species, although invisibly to us. 



The products of the sexual cells show us clearty 

 enough that out of each kind of egg only its own 

 species of organism can be developed. Certainly it 

 is not so plain that, besides their visible micro- 

 scopical characters, the tissues and organic parts 

 are in possession of more general characters, 

 identical in all the differently -specialised tissues of 

 a single organism ; but we may infer the existence 

 of such latent characters, at least partly, from the 

 results obtained, in the case of plants, by grafting, 

 in the case of animals, by transplantation and 

 transfusion. 



In the case of plants one may graft a twig cut 

 from one tree upon the stem or lower part of 

 another tree of the same kind, and so bring about a 

 firm and lasting union between the two. In a short 

 time the corresponding tissues of the parts brought 

 into connection quietly unite. Thus from two 

 different individuals a single living organism may 

 be produced artificially. 



