INTRODUCTION 187 



they derive it as we know from individuals like themselves who 

 transmit it to them by means of reproduction ; and we may be sure 

 that if the entire species of the Hon or oak chanced to be destroyed 

 in those parts of the earth where they are now distributed, it 

 would be long before the combined powers of nature could restore 

 them. 



I propose then to show what is the method apparently used by nature 

 for forming, in favourable places and conditions, the most simply 

 organised living bodies and through them the most perfect animals ; 

 how these fragile animals, which are the mere rudiments of animality 

 directly produced by nature, have developed, multipHed and become 

 varied ; how at length, after an infinite series of generations, the 

 organisation of these bodies has advanced in complexity and has 

 extended ever more widely the animal faculties of the numerous 

 resulting races. 



We shall find that every advance made in complexity of organisation 

 and in the faculties arising from it has been preserved and transmitted 

 to other individuals by means of reproduction, and that by this pro- 

 cedure maintained for very many centuries nature has succeeded 

 in forming successively all the living bodies that exist. 



We shall see, moreover, that all the faculties without exception are 

 purely physical, that is, that each of them is essentially due to activities 

 of the organisation ; so that it will be easy to show how, from the 

 humblest instinct, the origin of which can be easily ascertained, 

 nature has attained to the creation of the intellectual faculties from 

 the most primitive to the most highly developed. 



My readers must not expect to find here a treatise on physiology : 

 the public is already in possession of excellent works of this character 

 in which I have few alterations to suggest ; but I must marshal 

 together the general facts and well-established fundamental truths on 

 this subject, because I find that their association leads to new light 

 which has escaped those who have occupied themselves with details, 

 and because this light clearly shows us what the bodies endowed with 

 life really are, why and how they exist, and in what manner they develop 

 and reproduce ; lastly, by what methods the faculties observed in them 

 have arisen, and been transmitted and retained in the individuals of 

 each species. 



If we wish to grasp the chain of physical causation which brought 

 living bodies into existence, we must pay attention to the principle 

 which I embody in the following proposition : 



It is to the influence of the movements of various fluids in the more 

 or less solid substances of our earth that we must attribute the forma- 

 tion, temporary preservation, and reproduction of all living bodies 



