1 30 MODERN IDEAS OF EVOLUTION 



the law long ago formulated by Harvey, Omne vivum 

 ex ovo a law which modern research has amply 

 [confirmed, showing that every animal, however com 

 plex, can be traced back to an egg, which in its 

 simplest state is no more than a single cell, though 

 this cell requires to be fertilised by the addition of 

 the contents of another dissimilar cell, produced 

 either in another organ of the same individual or in a 

 distinct individual. This process of fertilisation 

 Haeckel seems to regard as unnecessary in the lowest 

 forms of life ; but though there are some simple 

 animals in which it has not been recognised, analogy 

 would lead us to believe that in some form it is 

 necessary in all. Haeckel s monistic view, however, 

 requires that in the lowest forms it should be absent, 

 and should have originated spontaneously, though 

 how does not seem to be very clear, as the explana 

 tion given of it amounts to little more than the 

 statement that it must have occurred. Still, as a 

 1 dualistic process it is very significant with reference 

 to the monistic theory. 



Much space is, of course, devoted to the tracing 

 of the special development or ontogenesis of man, 

 and to the illustration of the fact that in the earlier 

 stages of this development the human embryo is 

 scarcely distinguishable from that of lower animals. 

 We may, indeed, affirm that all animals start from 

 cells which, in so far as we can see, are similar to 

 each other, yet which must include potentially the 



