GBOUNDS AND CONSEQUENCES OF EVOLUTION. 351 



the Darwinian principle is of the nature claimed. We 

 can only deny that it is a full and adequate explanation 

 of all the facts. 



But the most impressible period of life is the embry- 

 onic. To what an extent must requisite supplies during 

 ovarian and uterine existence condition the physiological 

 activities which are making the being what it is to be. 

 It is certainly quite conceivable that favorable conditions 

 should so accelerate embryonic development that higher 

 results should be reached at full term, or that unfavora- 

 ble conditions should so retard development that lower 

 results should be reached. The influence of the struggle 

 for existence upon the development of the embryo has 

 not been entirely overlooked by Darwin, but acceleration 

 or retardation as the consequence of a struggle maintained 

 by the parent in the outer world is a conception which 

 characterizes the derivative theories of Hyatt and Cope. 

 It really seems to have struck upon a more fundamental 

 and productive condition of derivative variation than the 

 struggle for existence. The latter is a remoter condition, 

 while the former exists close by the seat of operation 

 of efficient cause. It accounts for regress as well as 

 progress. It addresses itself to the tissue-making forces 

 at the time when the foundations of the tissues are being 

 laid and not when the organic structure has been already 

 cast in its mould. 



But now, independently of all external conditions, it 

 is conceivable that the organism may be the subject of 

 an inherent and unremitting nisus, a tendency, in spite 

 of obstacles, to accomplish certain results, and attain to 

 fitter conditions. It is our own conviction that here lies 

 the secret force which works out the multifarious phe- 



