liz THE ORIGIN OF SPECIES 29 
Fauna of the world : it is obvious that the defini- 
jions of these species can be only of a purely 
structural, or morphological, character. It is 
probable that seceageage would have avoided 
snowledge 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 
sastonished the less, the more conversant he becomes 
with her operations; but of all the perennial 
imiracles 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.! 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 generaliy believed 
that the original nucleus of the egg (the germinal vesicle) 
disappeared. 1893. ] 
