300 Darwin and Pangenesis. [July, 
taught to believe that “species” means two distinct types whose 
union is infertile. But whatever may be urged against the author’s 
speculations upon the causes of certain natural phenomena, no one 
who knows anything of his character as an observer and writer 
will receive his statements of facts otherwise than with the most 
implicit confidence. 
And now in regard to this apparently insurmountable difficulty 
of hybridism, the author tells us that, as a rule, “species,” that is, 
widely diverging types, are infertile with each other, but that there 
are not only cases where they are fertile, but as we descend in the 
scale of nature, the crossing of species under certain conditions 
actually increases fertility, and that the only escape from making 
this admission in many cases, is by reasoning in a circle and calling 
species varieties because they are fertile. Well, as we do believe 
the author’s statements of facts, and as we consider the terms 
“variety,” “species,” “genus,” &c., to have been introduced into 
natural history by man for the guidance of his own limited intellect, 
and to have no actual existence in nature, we are unable to find 
any rational objection to the broad principle laid down by the 
author and his predecessors holding similar views, that all new forms 
of life are and have been modified descendants of pre-existing ones. 
Nor have we ever been able to see any other rational mode of 
accounting for the progression of nature. It is, indeed, quite proper 
at all times that new philosophical theories should be received with 
“philosophical caution ;” it is perfectly just that persons who hold 
opinions which have been accepted as truths in the past, should 
require of those who desire to convert them to a new philosophical 
faith, a large amount of trustworthy evidence in its favour, and 
should insist upon the clearmg away of patent obstacles to its 
acceptance ; but, adopting one of the ordinary principles of juris- 
prudence, when such an amount of evidence has been advanced, 
and when obstacles which were previously deemed insurmountable 
have been shown not to be so, the onus of proof then falls upon 
those who have held the original faith, which may have been con- 
ceived in ignorance, and perpetuated by unreasoning philosophical 
conservatism. 
The facts contained in the work before us already show that 
great modifications in osteological and external structure, and great 
divergences in habit, may in a brief period be brought about by the 
changed conditions and artificial selection practised under domes- 
tication. These modifications are so great, that were it not for the 
fertility of the varieties when crossed, no naturalist would hesitate to 
class them as distinct species. It is also proved almost to a certainty 
that many domesticated “ breeds” which are fertile when intercrossed 
have descended from ancestors of different species. 
Again, a glance at the history of the past shows us that step by 
