470 THE GENERAL PRINCIPLES OF ZOOLOGY 



and the spermatozoon, which consist of germ plasm, are held to 

 contain potentially all the characteristics of the female and the 

 male respectively, except such as are due to the use or disuse of 

 parts by the individual. In all the fundamental characteristics 

 of the species the two kinds of germ plasm, male and female, 

 are alike ; hence the single individual which results from their 

 fusion will possess these characteristics ; but the combination of 

 the characteristics which differed in the two parents may give rise 

 to very different characteristics in the offspring. Thus variations 

 may arise upon which natural selection then proceeds to act. 



In the foregoing paragraphs we have epitomized the principal 

 theories which treat of the method in which the evolution or 

 descent of the existing organic world has been brought about, 

 and have referred but briefly to the reasons which go to main- 

 tain a belief in the theory of evolution itself. We may now note 

 some of the more important facts which bear upon this subject. 



The most striking evidence in favor of the theory of evolution 

 is palaeontological and morphological. Much of the palaeonto- 

 logical evidence has been noted in a preceding chapter. The 

 various classes of animals did not all appear on the earth at the 

 same time, but in succession from the lower to the higher at 

 intervals of almost inconceivable length ; there is no direct evi- 

 dence of this in the case of many invertebrate classes, but this is 

 due to the fact that the great mass of the oldest sedimentary 

 rocks have been so metamorphosed, so altered in structure, that 

 all fossil remains which they may have contained, have disap- 

 peared. Again, in the vertebrate classes there is a break in the 

 order of succession ; the Mammalia appear in the Triassic, while 

 the Aves are first found in the next later period, the Jurassic. 

 Now morphological evidence shows that the Aves and the Mam- 

 malia have both developed from the Reptilia ; they are both 

 highly specialized types, so that it is a matter of no importance 

 which appeared first. On the other hand, we must remember 

 that of all the vertebrate classes the birds are the least likelv to 

 be preserved as fossils, so that, although no remains have been 

 discovered earlier than the Jurassic, it is by no means impossible 

 that they may have existed earlier. The incompleteness of the 

 geological record and the numerous factors which tend to pre- 

 vent the formation of fossils have already been pointed out; 



