208 GEOLOGICAL BIOLOGY. 



history of organisms may be examined from either of two 

 points of view, {a) We may examine the embryonic and 

 ontogenetic course of differentiation of the individual, and, 

 adding the theory of descent with modification, apply the 

 laws of individual development to the building of a theoreti- 

 cal phylogenesis for the whole series of organisms. This is 

 the method of Zoology. (/;) Or, we may examine the fossil 

 remains of organisms which have appeared in geological his- 

 tory, and by comparative study of their characters, arrange 

 them in series according to their resemblances and differences, 

 and thus reconstruct the history of the organisms from the 

 observed order of their appearance on the globe. This is the 

 paleontological method. 



Embryos or Fossils; the Imperfection of the Evidence. — In the 

 first case the chief criteria upon which the history is built are 

 the changes taking place in the growing embryo, minute and, 

 generally, microscopic, and of great difficulty of study. This 

 method requires great use of imagination in the interpretation 

 of rudimentary traces of characters, is based necessarily upon 

 few examples, and those seen, mainly by single observers. 

 The results are of necessity highly theoretical, and, like all 

 hypotheses, should be regarded as of no value in the face of 

 facts to the contrary. 



In the second case the chief criteria are fossils, which 

 are the remains of the hard parts and, in most cases, of adult 

 forms, imperfectly preserved, presenting a very small per- 

 centage of the total variety of forms that must have lived. 

 In this method the imperfection of the evidence and the 

 fragmentary nature of the fossils are the chief sources of im- 

 perfect judgment. The hypothetical series erected may be 

 proven by the actual sequence of the forms themselves. The 

 species may be arranged in the wrong race, but actual suc- 

 cession is always distinctly indicated, and the filling of gaps 

 is readily known to be theoretical. The known affinities of 

 living organisms are also in evidence here, to prevent wild 

 hypotheses based upon rare and imperfect fossils. 



From either point of view the possibilities of error are 

 enormous, and the proportion of theory to knowledge is 

 large ; but at the same time it must be said that the two 



