ii DEVELOPMENT OF ANIMAL LIFE 137 



to-day, to which they could be compared, are but the 

 empty and fixed examples of infinitely plastic forms, 

 pregnant with an unlimited future, the common stock 

 of the echinoderms, molluscs, arthropods, and verte 

 brates. 



One danger lay in wait for them, one obstacle which 

 might have stopped the soaring course of animal life. 

 There is one peculiarity with which we cannot help 

 being struck when glancing over the fauna of primitive 

 times, namely, the imprisonment of the animal in a 

 more or less solid sheath, which must have obstructed 

 and often even paralysed its movements. The 

 molluscs of that time had a shell more universally than 

 those of to-day. The arthropods in general were pro 

 vided with a carapace ; most of them were crustaceans. 

 The more ancient fishes had a bony sheath of extreme 

 hardness. 1 The explanation of this general fact should 

 be sought, we believe, in a tendency of soft organisms 

 to defend themselves against one another by making 

 themselves, as far as possible, undevourable. Each 

 species, in the act by which it comes into being, trends 

 towards that which is most expedient. Just as among 

 primitive organisms there were some that turned 

 towards animal life by refusing to manufacture organic 

 out of inorganic material and taking organic sub 

 stances ready made from organisms that had turned 

 toward the vegetative life, so, among the animal 

 species themselves, many contrived to live at the 

 expense of other animals. VFor an organism that is 

 animal, that is to say mobile, can avail itself of its 

 mobility to go in search of defenceless animals, and 

 feed on them quite as well as on vegetables. So, the 



1 See, on these different points, the work of Gaudry, Essai de palton- 

 tologit philosophique, Paris, 1896, pp. 14-16 and 78-79. 



