242 THE NATURE AND ORIGIN OF LIFE 



in essential qualities. To say that two species differed 

 qualitatively represents something which may be translated 

 into quantitative language. 



When De Vries showed us sudden mutations produced in 

 Oenothera, many scholars gravely discussed the question 

 whether the step passed over is a limit of species, whether, 

 strictly speaking, we ought to consider the mutation to 

 have passed over the limits of the species. There might have 

 been sense in this before Transformism, but the definition 

 of species has no longer any philosophical value. 



We can take many points of view to estimate the discon- 

 tinuity separating two different species. There are evidently 

 as many as there are characters susceptible of separate de- 

 scription in the two species and of comparison as correspond- 

 ing with each other. Now, as we have seen, the choice of 

 such characters is absolutely a matter of fancy. 



Yet there is one phenomenon which enables us to com~ 

 pare at the same time all the characters of one species with 

 all the characters of another fecundation. In this act 

 we can put in presence of each other the hereditary patri- 

 mony of a male element of species A and the hereditary 

 patrimony of a female element of species B. If fecunda- 

 tion ensues, it warrants us in denning a neighbourhood of 

 the given species. Such a definition is worth more than 

 one based on the consideration of some single character 

 taken at random. Therefore the act of fecundation has 

 always been chosen as a criterion by those who seek to give 

 an absolute dennition of species. 



