200 SCIENCE PROGRESS 



The magnitude of Mendel's achievement can be appreciated 

 by calling to mind the acute intellects which have been foiled 

 by the problem. For a century the study of heredity has 

 remained a repellent mass of statistics, with scarcely more 

 discernible order than might be found in any chance collection 

 of facts ; and of the would-be student it might be said, " Quaesivit 

 cselo lumen ingemuitque reperta." And for half of that century 

 there has lain hidden a solution of the riddle which brings 

 these facts into an order so straightforward that a child might 

 learn it. 



We should have nothing to do with the Mendelian laws 

 here were it not that they have given singular meaning and 

 interest to certain details of cell structure which before were 

 a mere collection of unintelligent facts. To take things in their 

 proper sequence I will first state the laws of inheritance so far 

 as they concern us, and then consider the structural characters 

 which seem to be their material basis. 



The first Mendelian principle which concerns us is this : 

 that what is transmitted from generation to generation may be 

 analysed into certain qualities or characters — constant characters, 

 as Mendel calls them — each of which is a unit in heredity, 

 each of which, therefore, is capable of independent trans- 

 mission. Thus in peas are length of stem, character of 

 inflorescence, colour of seeds, flavour, and so on. Underlying 

 these characters — each of which is capable of being picked 

 out or put back by a breeder, forming a substrate on which 

 they are erected — there would seem to be a basal character 

 which is inalienable, and which the breeder cannot, at present at 

 any rate, touch. Thus, in the case of peas, what is of necessity 

 transmitted is the fundamental qualities of " plant " as opposed 

 to " animal," and of " pea " as opposed to other plants. To 

 proceed in Mr. Bateson's words : 



These [unit] qualities or characters whose transmission in 

 heredity is examined are found to be distributed among the 

 germ cells, or gametes, as they are called, according to a definite 

 system. This system is such that these characters are treated 

 by the cell divisions (from which the gametes result) as existing 

 in pairs, each member of a pair being alternative to the other 

 in the composition of the germ. Now, as every zygote — that is, 

 any ordinary animal or plant — is formed by the union of two 

 gametes [in the process of sexual fertilisation], it may either 

 be made by the union of two gametes bearing similar members 



