CHAPTER XXII 



ADAPTIVE RADIATIONS OF THE VERTEBRATES 



It is significant that no biological science has offered evidence 

 which in any way controverts the theory of evolution (stated 

 many years before controlled conditions were used in experi- 

 mental research) and that each has contributed positive evidence 

 for the theory. From these rather diverse sciences an evolutionary 

 tree has been worked out. Undoubtedly the major portion of the 

 evidence as to the course of evolution has come from palaeontol- 

 ogy, the other sources of evidence confirming the fossil story. 



]\Iany phylogenetic trees, particularly in the older books, are 

 shown as a large main stem with small branches on either side. 

 This pine tree analogy does not fit the evidence. The semi- 

 vining "Wandering Jew" is more illustrative. From an original 

 stem are given off many shoots which take root and grow. Each 

 in turn divides, and as the new branches grow the older stems 

 tend to die or become unproductive and fail to develop new 

 buds. The plant spreads in all directions, rapidly in some places, 

 more slowly in others, depending upon the viability of the shoot 

 and the environmental conditions. In time only the terminal 

 shoots are left, with a few old stems to show the lines of 

 growth. 



Similar have been the adaptive radiations of the vertebrates, 

 and as a result many of the ancestral stems are imperfectly 

 known. Eventually, perhaps, palaeontologists will find the "miss- 

 ing links" which complete the story; but as long as hiatuses ex- 

 ist there will be a fertile field for speculation. It is not true that 

 a knowledge of ancestral groups is lacking, but that there are 

 few completely generalized individual specimens. This does not 

 interfere with the weight of evidence, for the student is con- 

 stantly amazed at the great number of specimens which have 

 been found and prepared for study. 



It is incorrect to speak of any living group as being an- 



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