1875.| The Question of Organic Evolution. 1gt 
of one well-known negative instance,* it was assumed that 
all the individuals of a species—even though belonging to 
distinct varieties—were capable of producing fertile progeny, 
whilst sexual intercourse between members of different 
species resulted either in sterile offspring or in none at all. 
It is surprising how much was here taken for granted. 
Both these tests utterly failed in pra€tice. The physiolo- 
gical criterion, baseless as we now know it to be, was 
neither applied, nor indeed capable of application, in one 
case in a thousand. Who ever tried whether every two 
supposed species of some closely connected group of insects 
were or were not capable of producing prolific offspring ? 
Experiments of this nature, continued for a few generations, 
can only be carried out upon animals in captivity. Now it 
is well established that, under such circumstances, certain 
animals refuse to breed at all. Undoubted varieties have 
been found which will not propagate with the original race. 
Thus “the variety evolved in Paraguay from our domestic 
cat pairs no longer with its ancestral stock, nor does the 
tame European guinea-pig with the wild ancestral stock of 
Brazil.” Where, again, is the proof that the forms of dogs, 
known to be mutually reproductive, are all sprung from one 
common ancestry? ‘The balance of evidence rather shows 
*‘that in various parts of the world, and at various periods, 
wild species of the genus Canis have been domesticated, of 
which the crosses produce fertile progeny to an extent 
almost unlimited.” The orthodox zoologists of the schools 
of Linnzus and Cuvier thus reasoned in a circle, proclaim- 
ing that ‘‘ forms belong to the same species, because they 
may be fruitfully crossed, and because they may be fruitfully 
crossed they belong to the same species; and, on the other 
hand, because such and such forms, when crossed, produce 
no fertile progeny, they constitute different species, and 
because they are different species, they generate no fertile 
offspring.” 
Nor was the morphological test more happy. To decide 
what variations of form indicated mere “‘ sports,” sub-species, 
and varieties, and what, on the other hand, were grave and 
persistent enough to constitute species, was left to the tact 
or the prepossessions of each systematist. The results were 
as discordant as might be expected. ‘Thus, taking the true 
* That of the mule. It has since been established that the hare and the 
rabbit, two species quite as distinct as the horse and the ass, are capable of 
producing offspring which, after many generations, show no decline in fer- 
tility. Among birds, fruitful hybrids are known to be produced both amongst 
the warblers and the gallinaceous tribe. 
VOL. V. (N.S.) 2B 
