ILLUSTRATIVE OF NATURAL SELECTION. 161 
Yew naturalists will doubt that all these may and pro- 
bably have been derived from a common stock, and 
therefore it seems desirable that there should be a unity 
in our method of treating them; either call them all 
varieties or all species. Varieties, however, continually 
get overlooked ; in lists of species they are often alto- 
gether unrecorded; and thus we are in danger of 
neglecting the interesting phenomena of variation and 
distribution which they present. I think it advisable, 
therefore, to name all such forms; and those who will 
not accept them as species may consider them as sub- 
species or races. 
6. Species. — Species are merely those strongly 
marked races or local forms which when in contact 
do not intermix, and when inhabiting distinct areas 
are generally ‘believed to have had a separate origin, 
and to be incapable of producing a fertile hybrid 
offspring. But as the test of hybridity cannot be 
applied in one case in ten thousand, and even if it 
could be applied would prove nothing, since it is 
founded on an assumption of the very question to be 
decided—and as the test of separate origin is in every 
case inapplicable—and as, further, the test of non- 
intermixture is useless, except in those rare cases’ 
where the most closely allied species are found in- 
habiting the same area, it will be evident that we 
have no means whatever of distinguishing so-called 
‘true specics” from the several modes of variation 
here pointed out, and into which they so often pass 
by an insensible gradation. It is quite true that, in 
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