CHAP. XII The Life- History of Animals 197 



something to marvel at, the same is equally true of its 

 beginning. 



(J)) The Gastreea Theory. From the frequent, though 

 not universal occurrence of the two-layered gastrula stage in 

 the development of animals, Haeckel concluded that the 

 first stable form of many -celled animal must have been 

 something very like a gastrula. He called this hypothetical 

 ancestor of all many-celled animals a Gastreea, and his infer- 

 ence has found favour with many naturalists. Some of the 

 simplest sponges, polypes, and " worms " are hardly above 

 the gastrula level. 



{c) Recapitulation. When we take a general survey 

 of the animal series, we recognise that the simplest animals 

 are single cells, that the next simplest are balls of cells like 

 Volvox, and that the next simplest are two-layered sacs of 

 cells like the. simple sponges, polypes, and worms above 

 referred to. These represent the three lowest steps in the 

 evolution of the race. They are not hypothetical steps in a 

 hypothetical ladder of ascent, they are realities. 



When we take a general survey of the individual 

 development of many-celled animals, we recognise that all 

 begin as single egg-cells, and that the ova divide into balls 

 of cells, which become in most cases two -layered sacs of 

 cells. It is therefore evident that the first three chapters in 

 individual history are precisely the first three steps in racial 

 history. 



Von Baer, one of the pioneer embryologists in the first 

 half of this century, discerned that the individual life-history 

 was in its general course a recapitulation of the history of 

 the race. He recognised that even one of the higher 

 animals, let us say a rabbit, began at the beginning as a 

 Protozoon, that it slowly acquired the features of a primitive 

 Vertebrate, that it subsequently showed the character of a 

 young fish, afterwards of a young reptile, then of a young 

 mammal, then of a young rodent, finally of a young rabbit. 

 He confessed his inability to distinguish whether three very 

 young embryos, freed from their surroundings, were those 

 of reptiles, birds, or mammals. In stating Von Baer's 

 vivid idea of development as progress from the simple 



