v.] IN THE THEORY OF NATURAL SELECTION. 277 



differences in plants are much more difficult to distinguish from 

 those which are acquired, than in animals. 



There is no doubt about the occurrence of innate and 

 hereditary individual characters in animals, and we may find 

 an especially interesting illustration in the case of man. The 

 human eye can with practice appreciate the most minute 

 differences between individual men, and especially differences 

 of feature. Every one knows that peculiarities of feature 

 persist in certain families through a long series of generations. 

 I need hardly remind the reader of the broad forehead of the 

 Julii, the projecting chin of the Hapsburgs, or the curved nose 

 of the Bourbons. Hence every one can see that hereditary 

 individual characters do unquestionably exist in man. The 

 same conclusion may be affirmed with equal certainty for all 

 our domestic animals, and I do not see any reason why there 

 should be any doubt about its application to other animals and 

 to plants. 



But now the question arises, — How can we explain the 

 presence of such characters consistently with a belief in the 

 continuity of the germ-plasm, a theory which implies the re- 

 jection of the supposition that acquired characters can become 

 hereditary .'' How can the individuals of any species come to 

 possess various characters which are undoubtedly hereditary, if 

 all changes which are due to the influence of external conditions 

 are transient and disappear with the individual in which they 

 arose ? Why is it that individuals are distinguished by innate 

 characters, as well as by those which I have previously called 

 transient, and how can deep-seated hereditary characters arise 

 at all, if they are not produced by the external influences to 

 which the individual is exposed ? 



In the first place it may be argued that external influences 

 may not only act on the mature individual, or during its develop- 

 ment, but that they may also act at a still earlier period upon 

 the germ-cell from which it arises. It may be imagined that 

 such influences of different kinds might produce corresponding 

 minute alterations in the molecular structure of the germ- 

 plasm, and as the latter is, according to our supposition, 

 transmitted from one generation to another, it follows that such 

 changes would be hereditary. 



Without altogether denying that such influences may directly 



