1 877.] Evolution by Expansion. 2 7 
The tradition of the fixity of species has become so deeply 
rooted in the human mind that only time, overwhelming 
evidence, and the untiring efforts of such pioneers of thought 
as Darwin, will ever eradicate the belief in it. He has al- 
ready combated this belief with such success that he has 
considerably shaken the opinions on the subject of many 
eminent naturalists, while he has made more converts to the 
doctrine of Descent than is generally supposed. He was 
by no means the first naturalist to discard the old theory of 
Creation. He, moreover, disclaims to have been the first to 
have accounted for the gradual development of species by 
natural selection, stating that in this he was long preceded 
by Dr. Wells and Mr. Mathews. Owing, however, to the 
labour and thought he has expended in elaborating the 
theory, it will doubtless be always chiefly associated with 
his name. He considers natural selection to have been the 
main instrument in the formation of species, but allows 
that sexual selection, climate, geographical distribution, and 
other minor influences have had their share in the work. 
The striking similarity in appearance, habits, and struc- 
ture presented by many species of plants and animals first 
caused naturalists to doubt the “ Fixity of Species,” and to 
seek a reason for “ Unity of Type ” as it is called. According 
to Darwin, “ Nothing can be more hopeless than to attempt 
to explain this similarity of pattern in members of the same 
class, by utility or the doctrine of final causes. The hope- 
lessness of the attempt has been expressly admitted by Owen 
in his most interesting work on the Nature of Limbs. On 
the ordinary view of the independent creation of each being, 
one can only say that so it is ; — that it has pleased the 
Creator to construct all animals and plants on a uniform 
plan : but this is not a scientific explanation.” 
The most obvious way of accounting for the phenomenon 
is by concluding that species possessing similar character- 
istics are related to one another. This is the doctrine of 
descent. We may go further, and presume that families, 
genera, and classes presenting points of similitude are also 
related. 
But assuming the truth of these conclusions, it is evident 
that species vary and develop into other forms ; also that 
their development is progressive, as it is well known that 
each geological era is characterised by higher types of fauna 
and flora than the preceding one. 
There must be a cause for this gradual development of 
new forms. The agent that effects these changes is, ac- 
cording to the Darwinian theory, natural selection. 
