THE MEANING OF THE WORD SPECIES. 291 
The word “ species” in Biology has two significations ; 
the one based upon morphological, the other upon 
physiological considerations. 
A species, in the strictly morphological sense, is simply 
an assemblage of individuals which agree with one another, 
and differ from the rest of the living world in the sum 
of their morphological characters; that is to say, in 
the structure and in the development of both sexes. 
If the sum of these characters in one group is repre- 
sented by A, and that in another by A +7; the two 
are morphological species, whether n represents an 
important or an unimportant difference. 
The great majority of species described in works on 
Systematic Zoology are merely morphological species. 
That is to say, one or more specimens of a kind of animal 
having been obtained, these specimens have been found 
to differ from any previously known by the character or 
characters n; and this difference constitutes the defi- 
nition of the new species, and is all we really know 
about its distinctness. 
But, in practice, the formation of specific groups is 
more or less qualified by considerations based upon what 
is known respecting variation. It is a matter of obser- 
vation that progeny are never exactly like their parents, 
but present small and inconstant differences from them. 
Hence, when specific identity is predicated of a group of 
individuals, the meaning conveyed is not that they are 
all exactly alike, but only that their differences are so 
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