CHAPTER II 

 The Genealogical Basis of Organic Differentiation 



IN spite of the many millions of years that have elapsed since 

 the first appearance of life on earth, and even since the 

 arrival of organisms with definite forms transmitted from 

 generation to generation, so many simple types still persist 

 that we can obtain an impression of sequence in living beings, 

 complete from the initial forms onwards. This is one of the 

 most astonishing facts that confront naturalists. It is true that 

 innumerable secondary series connected with the complete 

 cycle have disappeared, but the chain itself has been left 

 sufficiently intact to render its reconstruction comparatively 

 easy. The unicellular organisms which seem to possess the 

 simplest form next to the " primeval slime ", in which life 

 can manifest itself, still abound in both the animal and 

 vegetable kingdoms. Among the very oldest fossils we find 

 not only bacteria but Globigerince and Orbulince, similar to 

 those which now float upon the surface of the seas far distant 

 from the coasts. We can trace sponges closely related to the 

 beautiful Hexactinellidse, with their skeletons of elegant 

 opalescent lacework, that are still dredged from the waters of 

 the coasts of Japan and the Philippines, and from the deepest 

 parts of our own seas ; we also find polyps ; segmented 

 forms which have been conserved, in their general character at 

 least, in the Limuli of the Moluccas, Japan, and the Antilles, as 

 well as in the Estherida?, Nebalidae, and in the species of Cypris 

 of our oceans or fresh waters ; and, finally, Lingula and other 

 unsegmented Brachiopods, which are highly modified 

 descendants of annelid worms. Echinoderms are also en- 

 countered — of very special types, it is true, and molluscs with 

 shells, already differentiated into the three present classes of 

 Cephalopods, Gasteropods, and Lamellibranchs, of which the 

 Octopus, Snail, and Oyster are the best-known present day 

 examples, so far removed from the primitive types that we 



