igo PRIMITIVE FORMS OF LIFE 



lent itself to a series of modifications due to the immobility 

 which followed on a parasitic life or rendered necessary by 

 attitudinal alterations, themselves due to chemical phenomena 

 such as the secretion of lime or fatty substances, or to a 

 physiological phenomenon — the. great development of the 

 nervous system, followed by tachygenesis. Thus arose such 

 specializations as Flat Worms, Echinoderms, Molluscs, and 

 Vertebrates. The initial vertebrate forms, becoming degraded 

 as a result of their early fixation, constituted that peculiar 

 branch of the Animal Kingdom, the Tunicates. In all the varied 

 circumstances under which these animal modifications arose, 

 the organism, far from succumbing to unfavourable con- 

 ditions of existence, defended itself with success. It became the 

 artisan of its new organization, and, so to speak, created itself 

 anew by continuous efforts that could only be crowned with 

 success under conditions of absolute security. Natural selection 

 and the struggle for existence, as Darwin understood these 

 principles, had nothing to do with the creation of the great 

 organic types. Animals did, indeed, struggle for their existence, 

 but against the unfavourable conditions of their environment 

 and by reacting upon themselves, without having to fear 

 competition. They played an active part in their trans- 

 formations. How, indeed, could we suppose that animals 

 endowed with sense organs for receiving sensations, with nerve- 

 centres for their appreciation and reflexion to the periphery 

 in order to set the muscles and glands in motion, could remain 

 passive in the presence of incessant stimuli coming from 

 without ? 



It was only later, when the chief differentiations into type 

 had already taken place, that competition made its appearance 

 on the over-populated sea-shores. Its first consequence was not 

 combat but flight — toward the open sea and the ocean depths, 

 the fresh waters, and terra firma. The urge toward the land was 

 especially productive of transformations. In so far as the animal 

 remained aquatic, the local multiplication of epidermal cells 

 were produced externally as easily as internally, and gave 

 rise to superficial prominent appendages utilized in respiration. 

 From the time the animal organism became terrestrial this 

 multiplication was able to give rise only to internal tegumentary 

 appendages — the tracheal sacs of the Arachnida and the 

 tracheae of the Myriapods and Insects. Lungs took the place 



