INTRODUCTION. 5 



becomes more firmly established. If we can imagine 

 a time arriving when all the possible phenomena are 

 known, and the existing hypothesis still explains 

 them, nothing henceforth can overturn it, the science 

 is completed. That is the simple case in which a 

 theory has been victorious ; but if it is contradicted 

 by a single well-authenticated fact it must fall or 

 become modified. The more things a theory explains 

 in the present the more chance it has of success in 

 the future. It is still only a matter of chances, for 

 the theory is always at the mercy of unforeseen 

 observation, which may rudely overthrow it. 



There is no theory which must not be modified 

 constantly, at least in its details. To render it more 

 and more general by successive improvements is the 

 aim to be pursued. A collection of studies consti- 

 tutes a science when a hypothesis has arisen already 

 sufficiently strong to oblige us to refer to it all new 

 acquisitions, and to compel us to see if they fortify or 

 oppose it. 



It would indeed be a narrow conception if we were 

 to consider as scientific the partisans of the theory 

 alone ; more than anywhere else discussion is fruitful 

 in the natural sciences ; and if it is necessary to be 

 constantly preoccupied with the general ideas of the 

 day, it is not at all necessary to adhere to them 

 servilely. The naturalists of to-day are in possession 

 of a formula with which we must always preoccupy 

 ourselves ; in other words, there are natural sciences. 



The theory of Evolution. — -This hypothesis which 

 comes before all others is the theory of evolution. 

 This is not the place to expound it, to go over 

 the proofs which have been amassed to build it 

 up, nor the criticisms which have been directed 



