THE EVOLUTION OF LIFE 



of certainty) of a purely descriptive science, when it 

 confines itself to the faithful description of the directly 

 observed facts, either the embryonic processes in the 

 womb or the later metamorphic processes. The task of 

 phylogeny is much more difficult, as it has to decipher 

 long-past processes by means of imperfect evidence, 

 and has to use its documents with the utmost prudence. 



The three most valuable sources of evidence in 

 phylogeny are paleontology, comparative anatomy, and 

 ontogeny. Paleontology seems to be the most reliable 

 source, as it gives us tangible facts in the fossils which 

 bear witness to the succession of species in the long 

 history of organic life. Unfortunately, our knowledge 

 of the fossils is very scanty and often very imperfect. 

 Hence the numerous gaps in its positive evidence have 

 to be filled up by the results of two other sciences, 

 comparative anatomy and ontogeny. I have dealt fully 

 with this in my Anthropogeny. As I have also spoken of 

 the general features of these phyletic evidences in the 

 sixteenth chapter of the History of Creation, I need do 

 no more here than repeat that it is necessary to make 

 equal and discriminating use of all three classes of 

 documents if we are to attain the aim of phylogeny 

 correctly. Unfortunately, this necessitates a thorough 

 knowledge of all three sciences, and this is very rare. 

 Most embryologists neglect paleontology, most paleon- 

 tologists embryology, while comparative anatomy, the 

 most difficult part of morphology, involving most ex- 

 tensive knowledge and sound judgment, is neglected 

 by both. Besides these three sources of phylogeny there 

 is valuable proof afforded by every branch of biology, 

 especially by chorology, cecology, physiology, and bio- 

 chemistry. 



Although there has been very extensive phylogenetic 

 research during the last thirty years, and it has yielded a 

 number of interesting results, many scientists still seem 



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