August 28, 1914] 



SCIENCE 



289 



Wollaston^ — these however are only frag- 

 ments in comparison. Darwin regarded 

 variability as a property inherent in living 

 things, and eventually we must consider 

 whether this conception is well founded; 

 but postponing that inquiry for the pres- 

 ent, we may declare that with him began a 

 general recognition of variation as a phe- 

 nomenon widely occurring in nature. 



If a population consists of members 

 which are not alike but differentiated, how 

 will their characteristics be distributed 

 among their offspring? This is the prob- 

 lem 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 hered- 

 ity might be ascertained, the actual mechan- 

 ism by which the offspring is formed from 

 the body of the parent. In that endeavor 

 a noble pile of evidence has been accumu- 

 lated. 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 chromo- 

 somes of nearly related creatures may be 

 utterly different both in number, size and 

 form. Only one piece of evidence encour- 

 ages the old hope that a connection might 

 be traceable between the visible character- 

 istics of the body and those of the chromo- 

 somes. I refer of course to the accessory 

 chromosome, which in many animals dis- 

 tinguishes 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 



3 "On the Variation of Species," 1856. 



unpaired or accessory. The distinction 

 may be present or wanting, like any other 

 secondary sexual character. Indeed, so 

 long as no one can show consistent distinc- 

 tions between the cytological characters of 

 somatic tissues in the same individual we 

 can scarcely expect to perceive such dis- 

 tinctions between the chromosomes of the 

 various types. 



For these methods of attack we now sub- 

 stitute another, less ambitious, perhaps, be- 

 cause less comprehensive, but not less direct. 

 If we can not see how a fowl by its egg and 

 its sperm gives rise to a chicken or how 

 a sweet pea from its ovule and its pollen 

 grain produces another sweet pea, we at 

 least can watch the system by which the 

 differences between the various kinds of 

 fowls or between the various kinds of sweet 

 peas are distributed among the offspring. 

 By thus breaking the main problem up into 

 its parts we give ourselves fresh chances. 

 This analytical study we call Mendelian 

 because Mendel was the first to apply it. 

 To be sure, he did not approach the prob- 

 lem by any such line of reasoning as I have 

 sketched. His object was to determine the 

 genetic definiteness of species; but though 

 in his writings he makes no mention of in- 

 heritance it is clear that he had the exten- 

 sion in view. By cross-breeding he com- 

 bined the characters of varieties in mongrel 

 individuals and set himself to see how these 

 characters would be distributed among the 

 individuals of subsequent generations. 

 Until he began this analysis nothing but 

 the vaguest answers to such a question had 

 been attempted. The existence of any 

 orderly system of descent was never even 

 suspected. In their manifold complexity 

 human characteristics seemed to follow no 

 obvious system, and the fact was taken as 

 a fair sample of the working of heredity. 



Misconception was especially brought in 

 by describing descent in terms of "blood." 



