CHAPTER TWENTY-THREE 



Dominant 

 groups of 



Abundance 



of success- 

 f ol types 



THE PHYLA OF ANIMALS 



COULD we assemble together specimens of all the 

 kinds of animals which have ever existed, the gaps 

 which separate the phyla, classes, orders, and families 

 would be filled by what we now call "missing links." 

 Nevertheless, it would still be possible to distinguish 

 the larger divisions, since the animals possessing their 

 special characters would be much more numerous than 

 the intermediate forms. We may illustrate the facts to 

 a certain extent by comparison with objects made by 

 man, which have undergone a kind of evolution, though 

 by psychical instead of physical reproduction. No one 

 doubts, for example, that the wheels of a locomotive and 

 an automobile are alike modified forms of the original 

 cart wheel. It would be possible to accumulate a col- 

 lection illustrating numerous intermediate stages ; yet 

 if all wheels were to pass us in review, the highly adapted 

 ones would be vastly more numerous than those leading 

 up to them. As long as the automobile was in a rela- 

 tively experimental stage, the number of these machines 

 was comparatively small. As soon as the evolution had 

 gone far enough to produce a highly serviceable ma- 

 chine, the number enormously increased. So, then, 

 with the phyla of animals. The arthropod type, the 

 vertebrate type, etc., represent successful mechanisms, 

 which have increased and become diversified because 

 competent to do so. The "missing links" represent 

 Nature's experiments, perhaps well suited to particular 

 times and conditions, but not able to occupy any large 

 place in the world. A phylum (plural phyla) is the 

 largest division of the animal kingdom. Most people 

 think of animals as belonging to two great groups, the 



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