II THE ORIGIN OF SPECIES 29 



Fauna of the world : it is obvious that the defini- 

 tions of these species can be only of a purely 

 structural, or morphological, character. It is 

 probable that naturalists would have avoided 

 much confusion of ideas if they had more fre- 

 quently borne the necessary limitations of our 

 knowledge in mind. But while it may safely be 

 admitted that we are acquainted with only the 

 morphological characters of the vast majority of 

 species the functional or physiological, peculiari- 

 ties of a few have been carefully investigated, and 

 the result of that study forms a large and most 

 interesting portion of the physiology of reproduc- 

 tion. 



The student of Nature wonders the more and is 

 astonished the less, the more conversant he becomes 

 with her operations ; but of all the perennial 

 miracles she offers to his inspection, perhaps the 

 most worthy of admiration is the development of 

 a plant or of an animal from its embryo. Examine 

 the recently laid egg of some common animal, 

 such as a salamander or newt. It is a minute 

 spheroid in which the best microscope will reveal 

 nothing but a structureless sac, enclosing a glairy 

 fluid, holding granules in suspension. 1 But strange 

 possibilities lie dormant in that semi-fluid globule. 

 Let a moderate supply of warmth reach its watery 

 cradle, and the plastic matter undergoes changes 



1 [When this sentence was written, it was generally believed 

 that the original nucleus of the egg (the germinal vesicle) 

 disappeared. 1893.] 



