736 



SCIENCE. 



[N. S. Vol. IV. No. 99. 



fostered by selection. In this case, then, 

 selection will choose between the little 

 more complexity that is advantageous and 

 the little less complexity that is disadyan- 

 tageous. The little less complexity will be 

 eliminated, the little more complexity will 

 survive. The little less and the little more 

 are, however, in the same line of develop- 

 mental swing. Hence, the variations dis- 

 coverable in fossil mammals in which tooth 

 development along special lines is in pro- 

 gress, will, on the hypothesis of selection, 

 be plus and minus along a given line; in 

 other words, the variations will be determi- 

 nate, and in the direction of special adap- 

 tation." 



Prof. Weismann adopts a similar position 

 in his recent paper on germinal selection. * 

 " By the selection alone," he says, " of the 

 plus or minus variations of a character is 

 the constant modification of that character 

 in the plus or minus direction determined. 

 * ^ * "VVe may assert therefore, in general 

 terms, that a definitely directed progressive 

 variation of a given part is produced by 

 continued selection in that definite direc- 

 tion. This is no hypothesis, but a direct 

 inference from the facts and may also be 

 expressed as follows : By selection of the 

 kind referred to, the germ is progressively 

 modified in a manner corresponding with 

 the production of a definitely directed pro- 

 gressive variation of the part." 



In his Romanes Lecture, Prof. Weismann 

 makes another suggestion which is valuable 

 and helpful and which, I think, may be 

 further developed and extended. He is 

 there dealing with what he terms 'intra- 

 selection,' or that individual plasticity to 

 which I have frequently made reference. 

 One of the examples that he adduces is the 

 structure of bone. " Herman Meyer," he 

 says,t "seems to have been the first to call 



*Monist, Jan., 1896, p. 268. 



t Romanes Lecture on The Effect of External Influ- 

 ences on Development, pp. 11, 12. 



attention to the adaptiveness as regards 

 minute structure in animal tissues, which 

 is most strikingly exhibited in the structure 

 of the spoDgy substance of the long bones 

 in the higher vertebrates. This substance 

 is arranged on a similar mechanical prin- 

 ciple to that of arched structures in gen- 

 eral; it is composed of numerous fine bony 

 plates so arranged as to withstand the 

 greatest amount of tension and pressure, 

 and to give the utmost firmness with a 

 minimum expenditure of material. But 

 the direction, position and strength of these 

 long bony plates are by no means congen- 

 ital or determined in advance ; they depend 

 on circumstances. If the bone is broken 

 and heals out of the straight, the plates of 

 the spongy tissue become rearranged so as 

 to be in the new direction of greatest ten- 

 sion and pressure ; thus they can adapt 

 themselves to changed circumstances." 



Then, after referring to the explanation, 

 by Wilhelm Roux, of the cause of these 

 wonderfully fine adaptations by applying 

 the principle of selection to the parts of 

 the organism in which, it is assumed, there 

 is a struggle for existence among each other, 

 Prof. Weismann proceeds to show * that " it 

 is not the particular adaptive structures 

 themselves that are transmitted, but only 

 the quality of the material from which 

 intra-selection forms these structures anew 

 in each individual life. * * * It is not the 

 particular spongy plates which are trans- 

 mitted, but a cell mass, that from the germ 

 onwards so reacts to tension and pressure 

 that the spongy structure necessarilj^ re- 

 sults." In other words it is not the more or 

 less definite congenital adaptation that is 

 handed on through heredity, but an innate 

 plasticity which renders possible adaptive 

 modification in the individual. 



This individual plasticity is undoubtedly 

 of great advantage in race progress. The 

 adapted individual will escape elimination 



*Eonianes Lecture, p. 15. 



