HARDWICKE'S SCIENCE-GOSSIP. 



241 



NEO-DARWINISM, 



By a. G. TANSLEY. 



II. — The Problem of Heredity. 



'i^-^^l J^T^^^^ -^^ general pheno- 

 mena of heredity 

 are well known. 

 Not only do specific 

 characters reappear 

 with absolute accu- 

 racy in every gene- 

 ration, but those 

 slight " individual 

 differences," which 

 characterise the 

 various individual 

 members of any 

 species are also very 

 frequently trans- 

 mitted to the de- 

 scendants of the 

 members in which 

 they first appeared, 

 for more or fewer 

 generations, and with greater or less intensity. In 

 the human race the curved nose of the Bour- 

 bons, and the projecting chin of the Hapsburgs 

 are examples which are known to all ; and every- 

 one can multiply instances to any extent from his 

 own personal experience. How are we to account 

 for these wonderful facts? It is sometimes said 

 that they are accounted for by a law of persistence, 

 that "like produces like." But this is no real 

 solution at all, it is merely a restatement of the 

 problem. The first step in a real solution is to 

 recognise that in ordinary cases of reproduction, a 

 single cell is separated from the body of the parent, 

 that it subsequently divides and redivides,* and that 

 by the growth and differentiation of the products of 



* The pcocese of fertilisation, which in the vast majority of 

 cases precedes segmentation, is here purposely left out of 

 account to avoid complicating the statement of the problem. 

 The statement is only literally true in cases of partheno- 

 genesis. 



No. 323. — November 1891. 



its division, it is developed into the body of the 

 offspring. These are ascertained facts, and form the 

 only secure foundation for further investigation. 

 Now come the questions, how is it posssible for a 

 single cell, often not more than fij, inch in diameter, 

 to contain all the complex potentialities of the body 

 of one of the higher animals or plants ? How does 

 it come to possess these potentialities? and lastly, 

 how is the adult organism developed from the cell 

 containing them? These are the real problems of 

 heredity, which we have to attempt to solve. In the 

 present state of science it is impossible to furnish 

 definite answers to these questions in terms of 

 ultimate physical and chemical forces. Nevertheless 

 we can arrive at some conceptions of the modus 

 operandi of hereditary transmission from a biological 

 point of view. We must premise that the first 

 question is the most difficult of all. We know that 

 the tiny cell, often invisible to the naked eye, actually 

 does contain these potentialities, but it is only in the 

 vaguest manner that we can conceive Jiozo they are 

 contained in its molecular structure. The second 

 question embodies the problem of the transmission of 

 hereditary tendencies from one generation to another ; 

 this we shall call shortly the " problem of transmis- 

 sion." The third question embodies the problem of 

 the development of the hereditary tendencies con- 

 tained in the germ- cell into the actual features of the 

 organism. This we may call the " problem of de- 

 velopment." These two problems are distinct, and 

 should not be confused, although they are intimately 

 connected. A complete theory of heredity must 

 furnish a solution of both. 



We must now consider the chief attempts at solu- 

 tion which have from time to time been made by 

 various biologists. In chronological order they stand 

 thus : — • 



I. Mr. Herbert Spencer's theory of "physio- 

 logical units " (" Principles of Biology," vol. i. 1863). 



M 



