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, 

 mav have through transformations merged into different 

 kinds? This is not merely an idle question, insoluble from 

 the verv nature of the case; for the present races of animals 

 have a lineage reaching far into the past, and the question 

 of fixitv 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 Linnaeus supposed? Have 

 thev 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 

 thev have not been formed bv 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 



