144 PRIMITIVE FORMS OF LIFE 



in all possible detail. It is a subject 1 that has given rise to much 

 discussion, and has served as the basis of many and diverse 

 theories, having such general descriptive titles as alternating 

 generation, digenesis, gene agenesis , metagenesis, etc. It will be 

 sufficient for me to indicate that these facts are explained by- 

 considerations analogous to those which have here permitted 

 us to give the first scientific explanation ever attempted of 

 the formation of the great organic types which Cuvier regarded 

 as irreducible, and which he called " embranchements" . 



To sum up, the causes that have determined the formation of 

 the four great branches of the Animal Kingdom, into which the 

 primitive organization of the Worms has been modified, can be 

 thus synthetized : — certain phenomena of a purely chemical 

 nature, such as the secretion of lime or of fat, by weighing down 

 or lightening the animal, or of a purely physiological order 

 giving to the development of the nervous system certain 

 advantages enabling it to advance more rapidly than the other 

 ■organs, have determined a change of orientation in relation 

 to the ground. Either through some reflex action, or more or 

 less consciously, the animals that have undergone this change 

 in orientation have utilized such means as they possessed 

 within themselves, and especially their own muscles, to modify 

 their structure and bring about the greatest possible adaptation 

 of their organism such as it resulted from their former mode 

 of life, and the new conditions of existence imposed upon 

 them. In harmony with the general ideas of Lamarck's doctrine, 

 these animals have been the active agents of their own trans- 

 formation. For them this transformation was a period of crisis, 

 analogous to that which becomes so acute in insects which 

 undergo complete metamorphosis, and obliges them to shelter 

 themselves so carefully throughout its duration. They would 

 certainly have succumbed during this critical period if, at its 

 outset, their rivals had been very numerous and the struggle 

 for life very intense. It is therefore in an epoch when the com- 

 petition between them was not fierce, that is to say, very early 

 indeed, that these differentiations within the Animal Kingdom 

 must have begun. The struggle for existence which, as Darwin 

 showed in his celebrated books, played so important a role in 

 the choice of the secondary modifications characteristic of 

 the forms that have come down to us, and in the formation 



1 XLIII, 231. 



