304 THE ZOOLOGIST. 



might be ascertained, the actual mechanism by which the offspring 

 is formed from the body of the parent. In that endeavour 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 analyse 

 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 traceable 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, cannot 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 character. Indeed, so long as no one can show consistent 

 distinctions 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 substitute another, less 

 ambitious, perhaps, because less comprehensive, but not less direct. 

 If we cannot 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 our- 

 selves fresh chances. This analytical study we call Mendelian, 

 because Mendel was the first to apply it. To be sure, he did not 

 approach the problem 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 extension in view. By cross- 

 breeding he combined the characters of varieties in mongrel indi- 

 viduals, and set himself to see how these characters would be dis- 

 tributed among the individuals of subsequent generations. Until he 

 began this analysis nothing but the vaguest answers to such a ques- 

 tion 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." The common speech uses expressions such as 

 consanguinity, pure-blooded, half-blood, and the like, which call up a 

 misleading picture to the mind. Blood is in some respects a fluid, 

 and thus it is supposed that this fluid can be both quantitatively and 

 qualitatively diluted with other bloods, just as treacle can be diluted 



