16 SEA-SHORE LIFE 



trilobites, more than 1700 species of wliich existed in the Cambrian 

 and Silurian rocks, survives to-day ; and the sea lilies, which once 

 grew in vast colonies along our shores, are now represented by only 

 a few rare species in the deep ocean. 



The older naturalists did not realize the vastness of the destruc- 

 tion which this baUle for life has wrought, and thought that each 

 species was a separate creation that had existed unchanged since 

 the beginning. We now know that species appear to be distinct 

 one from another because the intermediate forms have died out; 

 and the longer a once closely related group of species has existed, 

 the wider do their differences become until we may look in vain 

 for the "linking forms" which once connected all of them. 



For example, we know that the vertebrates are recent, and are 

 descended from the far more ancient invertebrates. Accordingly, 

 we find that there are but few breaks in the chain of forms rang- 

 ing from the lowest fishes to the mammals. Indeed, the eml)ryos 

 of the highest mammals display gill slits in their necks, which soon 

 close ov-er, but which were of nse throughout life in their fish-like 

 ancestors. When we study the invertebrates, however, we find 

 wide gaps so that no one has yet been able to determine the rela- 

 tionships of some of the greatest groups. 



For example, we can not tell how the sponges, jellyfishes, eclii- 

 noderms or worms may be related one to another, but on the other 

 hand we have reason for the belief that crustaceans and insects are 

 descended from worm-like ancestors, and that moUusks may pos- 

 sibly have had a somewhat similar origin. 



It is interesting to observe that we meet with the same condi- 

 tions among plants. The algte and fungi show -wide gaps which 

 prevent one from arranging them in any connected series, whereas 

 the flowering plants, which are certainly more recent and have 

 descended from non-flowering forms, can be arranged in a fairly 

 regular chain, leading gradually from the lowest to the highest. 



We must bear in mind, however, that recent studies by Bate- 

 son, de Vries and others tend to show that new species may appear 

 suddenly as the offspring of old and well known forms; and that 

 these new and peculiar animals or plants may, through inheritance, 

 perpetuate their new peculiarities. There may then be no inter- 

 mediate or "linking" forms between the old and the new species. 



