The American 



[\,,-: 



the preservation of favorable variations. And the area of hab- 

 itation on the single plane of the sea bottom is so restricted as 

 compared with that within the many planes of oceanic waters, 

 that the struggle for place and food must have been greatly 

 increased, and the development and preservation of newly 

 adapted forms have been more rapid in consequence. 



This may seem to bring us to the very verge of the kingdom 

 of life as it is known to us from the oldest fossils yet discovered. 

 Yet in truth we are probably still remote from it. We are 

 still dealing with soft bodied animals, not with those possessed 

 of the hard external skeletons from which fossils are produced. 

 There is no good reason to believe that mere contact with the 

 earth induced the previously naked swimmers to clothe them- 

 selves in solid shells. In truth, the earliest bottom-dwellers 

 may have long continued soft bodied, the hard case or shell 

 being only slowly evolved. The mantle of the mollusk, for 

 instance, with its shell-secreting glands, is not likely to have 

 been a primary accessory of molluscan organization. The 

 same may be said of the chitin-forming glands of the Crusta- 

 cea, and the analogous glandular organs of other types. Such 

 conditions must have developed slowly, and their appearance 

 was probably due to an exigency of equally slow unfold- 

 ment. 



For now we come to another highly important problem, that 

 of the true disposing cause of the development of dermal 

 skeletons, on which there exists some basis for speculation. 

 In truth the fossils preserved for us in the Cambrian rocks have 

 an interesting tale to tell which has a strong bearing upon the 

 story of animal evolution. And this is, that all these bottom- 

 dwelLers, with the exception of the burrowing annelids, became 

 covered with what was probably defensive armor. They all 

 seem to have sought protection in one way or other, and in so 

 doing became in a measure degenerated forms of life, their 

 former ease of motion being now partly or wholly lost. 



All this represents an interesting stage in the process of 

 evolution, and indicates some special exigency in life condi- 

 tions which the animals of that age could only meet by ren- 

 dering themselves heavy and sluggish with a weight of inclosing 



