vi PRIMARY FACTORS OF ORGANIC EVOLUTION. 



have written on one part or another of the subject. If some very 

 meritorious essays have not been cited, it has been generally be- 

 cause I have confined myself to those in which facts or doctrines 

 were first presented, and have not had so much occasion to refer to 

 those of later date. 



Mr. Romanes, in his posthumous book, Volume II. of his Dar- 

 win t and After Darwin on Post- Darwinian Questions, expresses 

 the following opinion of the position which has been taken by the 

 Neo-Lamarckians of this country. He says that they do not dis- 

 tinguish between the " statement of facts in terms of a proposition, 

 and an explanation of them in terms of causality." Had Mr. Ro- 

 manes been acquainted with the literature of the subject published 

 in America and elsewhere during the last three years, he would 

 have had reason to change this view of the case. I think he would 

 have found in it demonstration "in terms of causality." 



At the outset it must be stated that a knowledge of the history 

 of organic evolution rests primarily on the science of morphology, 

 and secondarily on the kinematics of the growth of organic struc- 

 tures. The phenomenon to be explained is the genealogical suc- 

 cession, or phylogeny of organisms ; and the access to this subject 

 is through the sciences of paleontology and embryology. The phe- 

 nomena of the functioning of the organism, or physiology, are only 

 incidentally referred to, as not the real object of inquiry. Since 

 organic species are much more numerous than the tissues of which 

 they are composed, organogenesis must claim attention more largely 

 than histogenesis. It is true that histogenesis is fundamental, but 

 it is a science as yet in its early infancy, and little space can be 

 given to it. The exact how of organic evolution will never be 

 finally solved, however, until our knowledge of histogenesis is com- 

 plete. 



The research depicted in the following pages has proceeded on 

 the assumption that every variation in the characteristics of organic 

 beings, however slight, has a direct efficient cause. This assump- 

 tion is sustained by all rational and philosophical considerations. 

 Any theory of evolution which omits the explanation of the causes 

 of variations is faulty at the basis. Hence the theory of selection 

 cannot answer the question which we seek to solve, although it 

 embraces an important factor in the production of the general re- 

 sult of evolution. 



In the search for the factors of evolution, we must have first a 

 knowledge of the course of evolution. This can only be obtained 



