HARDWICKE'S SCIENCE-GOSSIP. 



267 



different parts of the idioplasm ; when any part has 

 become active, the tension between that part and the 

 rest becomes such that further growth of that part 

 cannot take place, but the next part begins to 

 grow actively. 



The idioplasm is composed of germr, (' Anlagen ') of 

 the different structures, each of which, reacting under 

 the stimulation of the growth of the previous ones, 

 becomes active in its turn and produces the corre- 

 sponding structure. This is very like Darwin's 

 conception of the development of gemmules. When 

 all the stages of ontogeny have been passed through, 

 the idioplasm returns to its original state — never 

 having differed from it except in respect of "condi- 

 tions of tension and movement " — and produces germ- 

 cells. This is the theory of the "cyclical develop- 

 ment of the germ-plasm " and reminds one of Mr. 

 Spencer's views on physiological units, which are 

 also apparently fundamentally the same in different 

 parts of the body. If, however, there is such a 

 substance as idioplasm, it is to be found in the cell 

 nuclei, as we shall see hereafter, and cannot be 

 conceived of as forming a network throughout the 

 body. Furthermore it is impossible to conceive of 

 the difference between the idioplasm of, for instance, 

 a muscle-cell and a white blood-corpuscle as consist- 

 ing in a mere difference of " conditions of tension 

 and movement." Professor Nageli adopts the hy- 

 potheses of redevelbpment and preformation in 

 essence, though he considerably modifies former 

 conceptions. 



Professor Weismann's hypothesis of the continuity 

 of the germ-plasm differs from all former theories by 

 his complete adoption of the epigenetic explanation 

 of ontogeny, and of the explanation by continuity of 

 the germ-plasm of the problem of transmission. He 

 supposes that a substance (germ-plasm) with a 

 specific molecular structure is handed on from 

 generation to generation. Part of this, at the 

 commencement of segmentation, is separated off for 

 the building up of the bodily structure, the rest is 

 preserved unchanged and is eventually transmitted to 

 the sexual cells. That portion which is converted 

 into the idioplasm of the body undergoes an actual 

 change of molecular structure at each stage of 

 ontogeny. 



We are now in a position to arrange the various 

 theories of heredity we have considered in a table, 

 which shall show at a glance their kind of explana- 

 tion of the general problems of heredity, and as an 

 outcome of this their bearing upon the question of 

 the transmission of acquired characters. We see at 

 once from the table (p. 266) that (apart from Mr. 

 Spencer's and Professor Hackel's hypotheses) those 

 theories which explain the problem of transmission 

 by supposing that redevelopment of the specific 

 bearer of hereditary tendencies takes place in each 

 generation, involve the transmission of acquired 

 characters, while those which explain the problem of 



transmission by supposing that this specific substance 

 is continuous from generation to generation do not. 

 And it is obvious that this must be so. We can form 

 no clear conception of how it is possible for acquired 

 characters to be reproduced as such in the next 

 generation except by the aid of the hypothesis of re- 

 development. Mr. Spencer's and Professor Hackel's 

 theories do not furnish such a clear conception. And 

 the clearest expression of the hypothesis of re- 

 development is undoubtedly to be found in Mr. 

 Darwin's theory of Pangenesis ; for Professor Nageli's 

 theory does not present us with a sufficiently definite 

 and intelligible mechanism. 



The issue, then, seems at present really to be 

 between Pangenesis and a theory of continuity of 

 the specific hereditary tendency-bearing substance. 

 But we have seen that there are objections of the 

 gravest kind to the hypothesis of Pangenesis. It is 

 therefore incumbent upon us to examine more in de- 

 tail the theory of continuity as it has been expressed 

 by Mr. Galton and Professor Weismann. If any form 

 of it explains the general facts of heredity with ease 

 and certainty, and can be shown to be in accord with 

 the general tendency of research, we shall be justified 

 in accepting it, provisionally at least ; and we shall 

 be further justified in considering that this is, so far 

 as it goes, good evidence against the transmission of 

 acquired characters. It is perfectly true that the 

 transmission of acquired characters may take placc^ 

 and yet that we may be unable, at present, to arrive 

 at a clear conception of the mechanism by which it 

 is brought about ; but it is also true that if, in the 

 absence of direct affirmative evidence on the point, 

 and from quite other considerations, the theory 

 which excludes such transmission can be shown to be 

 more likely to be correct than the theory which 

 involves it, we are forced to consider this fact alone 

 as evidence which negatives its probability. 



ROSSENDALE RHIZOPODS. 



No. 7. 



NO Rhizopod belonging to the sub-order Filosa 

 has a more elegant and complex shell than 

 that of any of the seven species of Euglypha, which 

 is characterised by the colourless, transparent test, 

 being made up of oval plates, overlapping in such a 

 way as to form very definite and beautiful patterns, 

 and by being further ornamented by numerous spines 

 or hairs. Fortunately, some of the species are very 

 common and widely distributed, being found in the 

 ooze of most ponds and ditches, among Sphagnum, 

 and on dripping rocks with mosses and Algae. 

 During the short period I have devoted to the special 

 study of the Rhizopods, I have found hundreds of 

 individuals belonging to two species, Euglypha 

 alveolata, and E. cUiata. The former is the com- 

 moner species here, and is a very variable form, both 



N 2 



