140 ZOOLOGY 



lieve that had it not been for early aquatic life, we 

 should not have possessed this useful structure. Man 

 at birth is tailless, but the early embryo has a distinct 

 pointed tail. So with many other structures, the prin- 

 ciple applying to plants as well as animals. These facts 

 have led to the saying that the ontogeny (individual de- 

 velopment) repeats the phytogeny (race development or 

 evolution). This is largely a fact, yet it has been 

 exaggerated in some quarters, with grotesque results. 

 It is not necessary, for instance, to assume that a boy 

 must pass through a stage in which he is a howling 

 barbarian. 



Past life as 6. Another class of evidence is derived from Paleon- 

 fossiis tology, the study of fossils. Since the earliest known 



rocks (Cambrian) which contain well-preserved fossils 

 show us a highly developed invertebrate fauna, it is im- 

 possible to trace the origin of the major invertebrate 

 groups. In the case of the vertebrates we are more 

 fortunate, and in several instances series of forms have 

 been discovered, illustrating evolutionary progress. 

 For an account of two of the best of these, see the 

 chapter (pages 417 and 425) on the horse and the ele- 

 phant. The geological record, in spite of the large col- 

 lections obtained, remains extremely fragmentary. 

 Thus, although we know the later (Tertiary) history of 

 the mammals fairly well, their much longer Mesozoic 

 evolution is represented only by the most meager frag- 

 ments. There are innumerable "missing links" in all 

 groups, and we can never hope to complete the history 

 of life from fossil remains. At the same time, all we 

 know accords with the theory of evolution, and every 

 fresh discovery in some measure illustrates it. It must 

 not be supposed that the several phyla have steadily 

 progressed from lower (less complex) to higher (more 



