296 GENERAL ZOOLOGY 



grouped under the general term, Age of Invertebrates, from 

 the predominance of these forms of life. The era was 

 very long, undoubtedly to be reckoned in millions of years. 

 We have no knowledge of the exact time that the different 

 species appeared, nor of the exact length of time they existed. 

 Neither do we know from actual specimens all the stages of 

 evolution through which the early forms of life may have 

 passed in coming to the form and structure in which we 

 know their kin to-day, for the intermediate types have been 

 lost on account of one catastrophe or another in the history 

 of the world. 



In the beginning of the era, life was marine, as far as fossil 

 records indicate. The earliest fossils are of sponges, corals, 

 sea-lilies or crinoids (see p. 246), worms, brachiopods (see 

 p. 233), mollusks, and trilobites (see p. 153). Subsequently 

 land animals made their appearance in forms like the arach- 

 nids and the insects. Before the close of the era a class of 

 vertebrate animals, the fishes, had come into existence. The 

 accompanying map (Fig. 147) will give an idea of the probable 

 growth of the land area of North America during the period. 



Evidence from Embryology and Morphology. Frequently 

 the only way the zoologist may know of the kinship of cer- 

 tain groups is by studying their early stages of development. 

 Since all the animals composed of more than one cell repro- 

 duce at one time or another by means of eggs and sperma- 

 tozoa, we see that all animals, however great the differences 

 between the adults may be, are alike in being composed of 

 one cell at the beginning of development. But in some ani- 

 mals the number and complexity of the changes intervening 

 between the egg stage and the adult stage are few and sim- 

 ple in comparison with the changes taking place in others. 

 All those animals which have few organs show relatively 

 few changes in development, in comparison with those 

 animals which have numerous, complicated organs. Hydra, 



