68 EVOLUTION 



indigestible particle that lies in its path, is almost the 

 simplest form of definitely animal life that we can 

 conceive.* Then we have a simple type (the Monad) 

 with a single lash to use as an oar for locomotion. We 

 have animals (still microscopic) in which a number of 

 single cells with lashes (or cilia) are united in a 

 compound animal. Higher in the scale the cluster of 

 cells doubles in on itself (as a boy forces in one half of a 

 soft indiarubber ball upon the other), and the internal 

 layer of cells does the work of digestion, the outer layer 

 the work of locomotion, for the whole. Higher still, 

 some of the cells specialise as germ or sex cells, and 

 some as sensitive cells. Then the sensitive cells gather 

 at the head, the digestive cells only line the inner cavity 

 (or stomach), and the other groups of cells take up the 

 special tasks of locomotion, excretion, and so on. 



We find strong reason to think that this was the main 

 line of the evolution of the animal body, when we turn 

 to the second piece of indirect evidence to which I 

 referred. It was known even before the time of Darwin 

 that all animals in their individual embryonic develop- 

 ment pass through a series of forms which more or less 

 reproduce the forms of their successive ancestors in past 

 time. The reproduction is not at all clear or complete 

 in the case of the higher animals, for the simple reason 

 that the embryonic life itself has in the meantime been 

 undergoing evolution ; besides that the series has to be 

 abridged for reasons of economy when it becomes very 

 long. But there is now a general agreement amongst 



* I hinted that the Amoeba is not so simple a matter as is 

 often imagined. Its plasm has a good deal of structure (though 

 no permanent organs) under a high power of the microscope 

 (one-twelfth and upwards), and it secretes a kind of acid 

 ( gastric juice") to digest its food in its temporary stomach 

 (or vacuole). 



