350 BIOLOGY AND ITS MAKERS 
animals, there is entombed in the rocks a great multitude 
of fossil forms which lived centuries ago, and many of which 
have become entirely extinct. How shall this great diversity 
of life be accounted for? Has the great variety of forms 
existed unchanged from the days of their creation to the 
present? Or have they, perchance, undergone modifications 
so that one original form, or at least a few original types, 
may have through transformations merged into different 
kinds? This is not merely an idle question, insoluble from 
the very nature of the case; for the present races of animals 
have a lineage reaching far into the past, and the question 
of fixity of form as against alteration of type is a historical 
question, to be answered by getting evidence as to their line 
of descent. 
Are Species Fixed in Nature?—The aspect of the matter 
which presses first upon our attention is this: Are the species 
(or different kinds of animals and plants) fixed, and, within 
narrow limits, permanent, as Linnzus supposed? Have 
they preserved their identity through all time, or have they 
undergone changes? This is the heart of the question of 
organic evolution. If observation shows species to be con- 
stant at the present time, and also to have been continuous. 
so far as we can trace their parentage, we must conclude that 
they have not been formed by evolution; but if we find 
evidence of their transmutation into other species, then there 
has been evolution. 
It is well established that there are wide ranges of varia- 
tion among animals and plants, both in a wild state and under 
domestication. Great changes in flowers and vegetables are 
brought about through cultivation, while breeders produce 
different kinds of pigeons, fowls, and stock. We know, 
therefore, that living beings may change through modification 
of the circumstances and conditions that affect their lives. 
But general observations extending over a few decades are 
