100 ON HEREDITY. 



insufficiently investigated cases, we may still maintain that the 

 assumption that changes induced by external conditions in the 

 organism as a whole, are communicated to the germ-cells after the 

 manner indicated in Darwin's hypothesis of pangenesis, is wholly 

 unnecessary for the explanation of these phenomena. Still we 

 cannot exclude the possibility of such a transmission occasionally 

 occurring, for, even if the greater part of the effects must be attri- 

 buted to natural selection, there might be a smaller part in certain 

 cases which depends on this exceptional factor. 



A complete and satisfactory refutation of such an opinion cannot 

 be brought forward at present : we can only point out that such 

 an assumption introduces new and entirely obscure forces, and that 

 innumerable cases exist in which we can certainly exclude all 

 assistance from the transmission of acquired characters. In most 

 cases of variation in colour we have no explanation but the survival 

 of the fittest l , and the same holds good for all changes of form 

 which cannot be influenced by the will of the animal. Very 

 numerous adaptations, such, for instance, as occur in the eggs of 

 animals, the markings, and appendages which conceal them from 

 enemies, the complex coverings which prevent them from drying 

 up or protect them from the injurious influence of cold,^must 

 have all arisen entirely independently of any expression of will, 

 or of any conscious or unconscious action on the part of the 

 animals. I will not mention here the case of plants, which as 

 every one knows are unconscious, for they are beyond my province. 

 In this matter, there can be no suggestion of adaptation depending 

 upon a struggle between the various parts of the organism (Roux) 2 . 

 Natural selection cannot operate upon the different epithelial cells 

 which secrete the egg-shell of Apus, since it is of no consequence to 

 the animal which secretes the egg-shell whether a good or a bad 

 shell is produced. Natural selection first operates among the off- 

 spring, and the egg with a shell incapable of resisting cold or 

 drought is destroyed. The different cells of the same individual 

 are not selected, but the different individuals themselves. 



In all such cases we have no explanation except the operation 

 of natural selection, and if we cannot accept this, we may as \\cll 



1 The colours which have been called forth by sexual selection must also be in- 

 cluded here. 



2 Wilhelin Roux, ' Der Kainpf der Theile im Organismus.' Leipzig, 1881. 



