INTRODUCTORY 13 



there is no reason to suppose that it can go on indefinitely. After 

 a time there is a return to sexual generation. 



We have now seen the first great step in evolution : the 

 cells, when they divide adhere together and specialise, the 

 result being a complex organism. Germ-plasm to continue 

 the race, is set apart in the reproductive cells. The eventual 

 death of the organism follows as a necessity from the division 

 of labour. 



Ill 



THE DEVELOPMENT OF THE INDIVIDUAL 



The ontogeny recapitulates briefly the phylogeny. The 

 development of the individual gives a brief resume of the 

 history of the evolution of the species. Unfortunately it is 

 not only brief, but somewhat confused : there are great blanks 

 in the story and there are later insertions. Much as we may 

 regret this, it is,only what was to be expected. The phylogeny 

 runs its course in the open world amid dangers and hardships : 

 whereas the embryo of the bird undergoes most of its develop- 

 ment within the egg, and that of the higher mammals in the 

 body of the mother. Hence we find membranes developed 

 which could not form part of any mature animal roughing 

 it in the world : in the chick two of them are netted with 

 blood vessels by means of which it breathes the air that 

 finds its way through the shell of the egg. In the higher 

 or placental mammals three membranes combine to form a 

 connection with the wall of the uterus and by means of this 

 connection, called the placenta, the embryo is nourished by 

 the blood of the mother. Then, too, in the avian egg and 

 that of the ornithorhyncus, the lowest of mammals, much has 

 been added to the minimum that constitutes an egg ; they have 

 departed from primitive simplicity. The distant marine ancestors 

 of mammals scattered their tiny eggs in the water. Then came a 

 great development of yolk for food and the membranes already 

 mentioned, then the reduction of the egg to quite a small size. 



In spite of all these causes of obscurity, the brief recapitulation 

 tells us a great deal. I will select a few. striking points, such 



