360 ANNUAL EEPOKT SMITHSONIAN INSTITUTION, 1915. 



theory of evolution had occupied the thoughts of many and found 

 acceptance with not a few before ever the " Origin " appeared. We 

 have come also to the conviction that the principle of natural selec- 

 tion can not have been the chief factor in delimiting the species of 

 animals and plants, such as we now with fuller knowledge see them 

 actually to be. We are even more skeptical as to the validity of that 

 appeal to changes in the conditions of life as direct causes of modi- 

 fication, upon which latterly at all events Darwin laid much em- 

 phasis. But that he was the first to provide a body of fact demon- 

 strating the variability of living things, whatever be its causation, 

 can never be questioned. 



There are some older collections of evidence, chiefly the work of 

 the French school, especially of Godron^ (and I would mention also 

 the almost forgotten essay of Wollaston -) . These, however, are only 

 fragments in comparison. Darwin regarded variability as a prop- 

 erty inherent in living things, and eventually we must consider 

 whether this conception is well founded; but postponing that in- 

 quiry for the present, we may declare that with him began a general 

 recognition of variation as a phenomenon widely occurring in nature. 



If a population consists of members which are not alike but dif- 

 ferentiated, how will their characteristics be distributed among their 

 offspring? This is the problem which the modern student of 

 heredity sets out to investigate. Formerly it was hoped that by the 

 simple inspection of embryological processes the modes of heredity 

 might be ascertained, the actual mechanism by which the offspring 

 is formed from the body of the parent. In that endeavor a noble pile 

 of evidence has been accumulated. All that can be made visible by 

 existing methods has been seen, but we come little if at all nearer to 

 the central mystery. We see nothing that we can analyze further — 

 nothing that can be translated into terms less inscrutable than the 

 physiological events themselves. Not only does embryology give no 

 direct aid, but the failure of cytology is, so far as I can judge, equally 

 complete. The chromosomes of nearly related creatures may be 

 utterly different both in number, size, and form. Only one piece of 

 evidence encourages the old hope that a connection might be trace- 

 able between the visible characteristics of the body and those of the 

 chromosomes. I refer of course to -the accessory chromosome, which 

 in many animals distinguishes the spermatozoon about to form a 

 female in fertilization. Even it, however, can not be claimed as the 

 cause of sexual differentiation, for it may be paired in forms closely 

 allied to those in which it is unpaired or accessory. The distinction 

 may be present or wanting, like any other secondary sexual char- 

 acter. Indeed, so long as no one can show consistent distinctions 



1 De I'Espdce et ties Races dans Ics Etres Organises, 1850. 

 - On the Variation of Species, 1856. 



