xv THE PHILOSOPHY OF ZOOLOGY 623 



appears in the offspring, there is, to begin with, great difficulty in 

 proving that it is really new : characters not present in the 

 parents are known to be frequently inherited from a more or 

 less remote ancestor. But, if we suppose it to be established 

 that the character is a new one (and absolutely new characters 

 must appear, or we should have no evolution), then it would 

 require a very accurate knowledge of all the circumstances to 

 enable us to be certain whether the appearance of the character 

 is not due to the action of some external influence on the parent, 

 either during development or in the adult state, rather than to a 

 change arising within the reproductive cells. Instances are 

 frequently brought forward which have been supposed to afford 

 evidence of the transmission of mutilations from parent to off- 

 spring ; but such a transmission must, from the nature of the case, 

 always be extremely difficult to prove, and the majority, at least, of 

 such cases are found, on a careful analysis, to be capable of other 

 interpretations. On the other hand, though well-established 

 cases of the inheritance of mutilations would greatly support the 

 doctrine that acquired characters are transmissible, the negative 

 results that have attended certain experiments on mutilation are 

 of little value in the direction of proving that extrinsic variations 

 cannot be transmitted, since, as has already been pointed out, such 

 experiments in mutilation cannot be said to reproduce the con- 

 ditions under which an extrinsic variation might be supposed 

 to be transmitted ; the mutilation is instantaneous ; the variation 

 must be supposed to be the result of long-continued action, which, 

 it might be expected, would have a sufficiently profound effect to 

 engraft it permanently on the organism. 



It should be pointed out here that there is no absolutely hard 

 and fast line to be drawn between the intrinsic and extrinsic 

 variations, since changes in the sexual cells may very well be due, 

 directly or indirectly, to influences exerted from without. The 

 material from which reproductive cells may subsequently be 

 fashioned is, in plants and in many animals, in such close and 

 intimate union so far as can be seen with the other proto- 

 plasmic elements of the organism, that it seems highly probable 

 that influences affecting the latter may in many cases affect also 

 the former. 



Another question that presents itself in connection with heredity 

 is, Can any special part of the germ-cell be fixed upon as the 

 part specially concerned in hereditary transmission ? Certain 

 experiments which have been made on the ova and sperms of 

 Sea-urchins have an important bearing on this question. It has 

 been found that an ovum artificially deprived of its nucleus will 

 develop if a sperm (consisting mainly of nuclear matter) be 

 introduced into it to take the place of the original nucleus. And, 

 further, it has been shown that if an ovum of one species be 



