The Principles of Biology, Volume 1 · Herbert Spencer
THE ARGUMENTS FROM EMBRYOLOGY.
Let it be remarked, finally, that the ability to trace out embryologic kinships and the inability to do this, occur just where, according to the hypothesis of Evolution, they should occur. We saw in § 100a that zoologists are agreed in grouping animals into some 17 phyla--_Mollusca_, _Arthropoda_, _Echinodermata_, &c.--each of which includes a number of classes severally sub-divided into orders, genera, species. All the members of each phylum are so related embryologically, that the existence of a common ancestor of them in the remote past is considered certain. But when it comes to the relations among the archaic ancestors, opinion is unsettled. Whether, for instance, the primitive _Chordata_, out of which the _Vertebrata_ emerged, have molluscan affinities or annelidan affinities, is still a matter in dispute. With regard to the origins of various other types no settled conclusions are held. Now it is clear that on tracing down each branch of the great genealogical tree, kinships would be much more manifest among the recently-differentiated forms than among those forms which diverged from one another in the earliest stages of organic life, and had separated widely before any of the types we now know had come into existence.
Ideas discussed in this chapter
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