GENERALISATIONS. 71 



ductive cells lay ; perhaps they were like the planula larvae 

 of some Coalentera two-layered, externally ciliated, oval 

 forms without a mouth. 



(3) The fact of recapitulation. It is a matter of experi- 

 ence that we recapitulate in some measure the history of 

 our ancestors. Embryologists have made this fact most 

 vivid, by showing that the individual animal develops along 

 a path the stations of which correspond to some extent with 

 the steps of ancestral history. 



(1) The simplest animals are single (i) The first stage of development 



cells (Protozoa). is a single cell (fertilised 



(2) The next simplest are balls of ovum). 



cells (e.g. Volvox}. (2) The next is a ball of cells 



(3) The next simplest are two- (blastula or morula). 



layered sacs of cells (e.g. (3) The next is a two-layered sac 

 Hydra}. of cells (gastrula). 



Von Baer, one of the pioneer embryologists, acknow- 

 ledged that, with several very young embryos of higher 

 Vertebrates before him, he could not tell one from the 

 other. Progress in development, he said, was from a general 

 to a special type. In its earliest stage every organism has 

 a great number of characters in common with other 

 organisms in their earliest stages ; at each successive 

 stage the series of embryos which it resembles is nar- 

 rowed. The rabbit begins like a Protozoon as a single 

 cell ; after a while it may be compared to the young 

 stage of a very simple vertebrate ; afterwards, to the young 

 stage of a reptile ; afterwards, to the young stage of almost 

 any mammal ; afterwards, to the young stage of almost 

 any rodent ; eventually it becomes unmistakably a young 

 rabbit. 



Herbert Spencer expressed the same idea, by saying that 

 the progress of development was from homogeneous to 

 heterogeneous, through steps in which the individual history 

 was parallel to that of the race. But Haeckel has illustrated 

 the idea more vividly, and summed it up more tersely, than 

 any other naturalist. His "fundamental biogenetic law' 

 reads : " Ontogeny, or the development of the individual, is 

 a shortened recapitulation of phylogeny, or the evolution of 

 the race." 



It is hardly necessary to say that the young mammal is 



