PRIMITIVE ANIMAL FORMS 125 



increasing embryogenetic acceleration, favoured by the 

 accumulation within the eggs of a larger and larger amount 

 of reserve nutritive material, leads to the higher forms that 

 develop entirely within the egg. These creatures hatch out 

 with all the segments they will ever have, and often in their per- 

 manent form. This is the necessary preliminary condition which 

 alone has permitted the realization of organisms capable of 

 living in fresh water, or on the earth, and hence of breathing air. 

 The two series of free and segmented animals have evolved 

 naturally at the same time as the Algae, the Sponges, the 

 Polyps and the Bryozoa. We may assume that from the 

 beginning the waters were peopled with the most diverse 

 forms, which could vary in many ways, according to circum- 

 stances, because they were not under the domination of 

 heredity and because, on the other hand, the struggle for 

 existence was not very intense and the mere ability to keep 

 alive sufficed to perpetuate their stock. All that was possible 

 was attained. It is due to this easy stage in the struggle 

 for existence that certain deformations, apparently dis- 

 advantageous, of primitive types have occurred and given rise 

 to forms which appear almost monstrous, but which, never- 

 theless, have managed to occupy a most important place in 

 nature. As from the point of view of locomotion, there are 

 only two kinds of existence, immobility, which means attach- 

 ment to some foreign body, and mobility we might suppose 

 that there should be only two types of structure for animals, 

 the branched type, linked with immobility, and the 

 segmented, linked with locomotion. There are, however, four 

 others : (1) the Echinoderms, radiate without being fixed ; 

 (2) the Molluscs, non-segmented, and often found in spiral or 

 helicoidal form ; (3) the Tunicates, fixed or swimming, but 

 unsegmented and non-radiate. This last is a regressive type, due 

 to degeneration following upon fixation to the earth of already 

 highly organized animals, and which were nothing less than 

 the precursors of the Vertebrates. Of these precursors 

 Amphioxus is the last representative ; (4) Finally we have the 

 Vertebrates, truly segmented, but with an internal structure 

 apparently quite different from the expected and theoretical 

 structure of segmented animals. Our task now is to find out 

 how such organisms were enabled to develop. 



