8 THE NATURE AND ORIGIN OF LIFE 



(Paleontology), we succeed in establishing between the two 

 certain ties of relationship and descent ; this is the special 

 science of the Origin of Species. 



The mere existence of " comparative " sciences (compara- 

 tive anatomy, physiology, and embryology) along with the 

 corresponding " descriptive " sciences is enough to prove that 

 the diversity of living forms is not accompanied by absolute 

 differences among the species. It allows us to constitute, 

 in our general catalogue of beings, more or less exten- 

 sive groups ; and, among the various members of each of 

 these groups, anatomical, physiological, or embryological 

 comparison may be fruitful. Thus it is useful to compare, 

 from the descriptive point of view, the dog, the horse, and 

 the rabbit, which are mammals ; again it is interesting to com- 

 pare the dog, cock, lizard, and trout, which are vertebrates ; 

 while it would be fanciful to compare the anatomy of the 

 rabbit and that of the sea urchin or earthworm. The latter 

 have nothing in common ; and the differences in anatomy 

 become more strongly emphasized, if possible, when we pass 

 from such animals to the chestnut -tree, the lily, and the 

 mushroom, which are vegetable. 



Yet man long since learned to unite under the common title 

 of living beings bodies so little resembling each other as the 

 dog, earthworm, fern and rosebush. Such bodies therefore 

 must not only have something in common, but this some- 

 thing in common must be easy to find, since it did not escape 

 men as ignorant as our ancestors. But we shall see that the 

 common opinions regarding life are, to a certain degree, 

 different from those which science establishes by means of 

 instruments of research more powerful than those natural to 

 man. 



It is Biology which studies the characters common to all 

 living beings, whether animal or vegetable characters 



