VARIATION. 31 



to ask, what is the relation between the quality of a seed and the 

 qualities of the plants ? What is the relation between the germ and 

 the individual which grows from it ? How does an organism come 

 to be as we finally see it, how does it develop its cha acters? 



If we contemplate an individual plant or animal, forgetting 

 for the moment that it ever had ancestors, we understand that 

 its present characters, the qualities which we see it have, are 

 the result of its development, of the way in which it grew up 

 from a germ. We see, that at every single moment of its devel- 

 opment an embryo has its own qualities, which are continually 

 changing in the course of this development. If we observe a 

 morula develop into a blastula we witness a new quality which 

 comes into existance, hollowness, or, we can say, we see the 

 birth of a new organ, the cavity. This new quality of the embryo 

 is the result of the development, of the multiplication of the 

 cells of the morula and of the migration of the cells to the 

 surface of the cell-agglomeration. 



If now we remember that the organism has its ancestors, 

 and that these ancestors, and the brothers and sisters, all 

 develop into blastulas out of morulas, we see that there must 

 be something common to the family, something common to 

 all the individuals of the group which makes this sort of pro- 

 cess happen in this way. We see that one or more of the causes 

 of this development must be inherited, must be common to 

 all the individuals of the group. But we also know quite' a 

 number of things which most certainly are also factors in the 

 development of all these animals, and which also certainly 

 are common to an enormous number of different sorts of organ- 

 isms, factors which influence the development from without, 

 such as the relation between salts in the sea-water in which the 

 embryos develop, or oxygen, or gravity, which are of vital im- 

 portance and whose cooperation to the development is abso- 

 lutely necessary for the life of an individual. And there are 

 others of relatively less importance, so that they may vary in 

 wide limits, or even be lacking altogether, without seriously 

 impairing the vitality of the individual. 



