THE METAZOA 



It is this great feature of progressive 

 development which has led biologists, from 

 Darwin down, to restrict their attention 

 to the metazoa. As we have already dis- 

 cussed the various theories propounded as 

 explanatory of the course of development, 

 we need not recapitulate them here. But 

 a great and hitherto insuperable diffi- 

 culty inexorably and equally attends all 

 these speculators from the very start. If 

 they had only full-grown plants and ani- 

 mals to deal with, the task would be com- 

 paratively easy. But, as we have seen, 

 each metazoan form of life begins as a 

 single microscopic cell, which is then more 

 itself and nothing but itself than it will 

 ever be afterward. In other words it looks 

 at first altogether like one of those eternal 

 unicellular things which never change nor 

 develop for millions of years. But that 

 which makes the vast difference between 

 this metazoan unicellular thing and the 

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