240 OILMAN A. DREW. 



tors and when their embryos are examined every one must be 

 struck with their close resemblances. These embryos would 

 seem to point to a free swimming ancestral form that obtained its 

 food by means of surface cilia. 



The first living forms that made their appearance on the earth 

 must have used non-living substances for food. What the nature 

 of these substances were, whether they were of a comparatively 

 simple nature, like those that are used by our green plants 

 to-day, or whether they were of an entirely different nature, we 

 have no means of knowing, but it is evident that their food was 

 not alive. 



Then came the discovery by some form that the protoplasm 

 of other forms could be used for food. This must have been the 

 first great factor that led to the competition of forms and called 

 for the improvement of bodily machinery among living things, to 

 aid in the struggle thus begun, the struggle to get food and to 

 escape from being used as food. As Professor Brooks has indi- 

 cated, 1 this would naturally lead to the discovery and colonization 

 of the bottom of the ocean because of the greater advantages it 

 offered both for capturing food and in affording means of protec- 

 tion. This introduces the further element into the competition, 

 of some positions being far more favorable than others, and as 

 the struggle for position increased, a struggle that has never 

 ceased, the competition, especially between close relatives, must 

 have become very severe. 



These factors, with the struggle dependent upon them, must 

 have caused changes in structure (in the improved machinery that 

 aids forms in getting food and in keeping from being used as 

 food) to change very rapidly and it seems very plausible that in 

 a comparatively short time in those days when forms were of 

 simple structure and this keen competition was begun, the founda- 

 tions of the great types of animal structure were laid. 



We know that among our earliest fossils are to be found both 

 lamellibranchs and gastropods, and it is back in the earlier time 

 that we must look for the changes that have resulted in the for- 

 mation of these classes. 



1 Brooks, " The Origin of the Oldest Fossils and the Discovery of the Bottom of 

 the Ocean," Smithsonian Report for 1894 (also Salpa). 



