HARDWICKE'S SCIENCE-GOSSIP. 



39 



II. Veins of parastigma forming an X. 

 HH. Labial palpi short, near together: Nemoura. 



Prothorax a little longer than wide ; meso and meta- 



thorax with central notch ; antennae yellow at base ; 



wings brownish grey, veins darker; Variegata. 

 Antennae wholly black ; wings white, clouded with 



grey : Meyeru 

 Prothorax as Wide as long, shining; wing veins edged 



with dark grey : Nitida, 

 Prothorax longer than wide ; head and antennse light 



brown ; feet pale : Cinerea. 

 Posterior femora wholly dark brown ; wings opaque 



with the base yellow: Humeralis. 

 Shining black ; prothorax rugose, with a dorsal fur- 

 row; legs and feet dark; wings brownish with 



darker veins : Sulcicollis. 

 Dark shining brown ; antennae with a slight pile ; feet 



pale ; wings semi-transparent, veins pale : Inco?i- 



spicita. 



(To be continued.} 



NEO-DARWINISM. 



By A. G. Tansley. 



IV. — The Hypothesis of Continuity apflied 

 to the Solution of the Problem of "Here- 

 ditary Transmission. 



w 



E must now consider more fully Mr. Galton's 

 and Professor Weismann's theories of 

 heredity — the two theories which explain the problem 

 of transmission by supposing that the substance which 

 is the specific bearer of hereditary tendencies is 

 continuous from generation to generation. And it 

 must be again insisted that Mr. Galton's theory is 

 not practically identical with Professor Weismann's, 

 as has been stated * ; nor is it a mere modification of 

 Mr. Danvin's, as has also been stated. + 



To put it briefly, it differs from the former by its 

 " preformational " character, and from the latter by 

 its substitution of continuity for redevelopment. 

 Hence, though it stands intermediate between these 

 two theories, it differs from both in important 

 respects. 



It occupies an extremely important place in the 

 development of thought on the question of the 

 mechanism of heredity, through having first stated in 

 a precise manner this idea of continuity. 



Mr. Galton's profound anthropological studies 

 convinced him that the phenomena of the trans- 

 mission of inherent or congenital characters were the 

 important phenomena of heredity which required 

 explanation, and this caused him to formulate the 

 hypothesis of the continuity of residual gemmules as 

 the main idea of his theory. Mr. Darwin, it is true, 

 was compelled to suppose that certain of his 

 gemmules remained latent for many generations, in 

 order to explain the facts of atavism, but the 

 phenomena which Pangenesis was especially devised 

 to explain were, as we have seen, the supposed 

 transmission of acquired characters. Mr. Galton, on 

 the other hand, while accepting the Pangenetic 



* Wallace's "Darwinism," p. 443. 



i" Poulton. Note in Weismann's "Essays on Heredity,' 

 p. 173; and Lloyd Morgan's "Animal Life and Intelligence, 1 

 p. 135- 



explanation of the few cases in which he thinks such 

 transmission probable, relies on the theory of con- 

 tinuity to explain the main facts of heredity. It is 

 obvious indeed that the assumption of the continuity 

 of a certain amount of germ-substance is necessary 

 to explain the latency of characters for one or more 

 generations. Darwin, as we have seen, recognised 

 this in his atavistic gemmules. But the question 

 which we have to face now is, whether this assump- 

 tion cannot and ought not to be carried farther, so as 

 to make it the central idea of our theory of hereditary 

 transmission. 



Mr. Galton goes so far as to say that it is 

 " indeed hard to find evidence of the power of the 

 personal structure to react upon the sexual elements 

 that is not open to serious objection;" and "we 

 might almost reserve our belief that the structural cells 

 can react on the sexual elements at all." Nothing 

 can be clearer than his recognition of the ability of 

 the theory of continuity to explain the main facts- of 

 heredity. 



Professor Weismann was led to exactly the same 

 conclusion from general biological evidence, but his 

 theory took a different form, partly from its having 

 been promulgated nine years later than Mr. Galton's 

 — during which time the ceaseless activity of research 

 had brought to light many new facts — and partly 

 from his attention not having been chiefly concen- 

 trated on anthropological phenomena. 



Mr. Galton conceives of the body as consisting of 

 " organic units," each of which he thinks must have 

 had a separate origin. Hence he conceives of the 

 germ substance (stirp), of every fertilised ovum as 

 consisting of an enormous number of gemmules, and 

 each " organic unit " of the body as being represented 

 by one or more of these gemmules. In this way 

 only does he conceive it possible to understand how 

 a child can inherit minute features, some from one 

 parent and some from the other (particulate inherit- 

 ance). But it is not clear that Mr. Galton is correct 

 in arguing from such phenomena to the existence of 

 separate organic " gemmules." It is doubtless true 

 that the separate " potentialities " (using this term 

 in its widest sense) of the various minute features 

 must exist, but since the features themselves are only 

 the final outcome of a long course of ontogenetic 

 development, it is quite possible that they may all 

 exist in the germ simply as differences of mutual 

 arrangement and as differences of motion of the 

 parts of a specific substance (the germ-plasm of 

 Weismann). Still, there is no doubt that Mr. 

 Galton's gemmules are very much easier to deal with, 

 and much clearer conceptions can be formed of the 

 manner in which they are supposed to behave. 

 Nevertheless, as we shall see presently, it seems on 

 the whole more probable that they do not really 

 exist, but that we must conceive of the " germ- 

 plasm " as containing the potentialities of the 

 organism. Admitting, however, for the present, the 



