204 THE PHYSIOLOGY OF REPRODUCTION 



^' bad " in the former and " good " in the latter in regard to mutton 

 production), segregation appeared very clearly in animals of both 

 the second and third generations, the points being reproduced in all 

 possible combinations among the cross-bred sheep. This case of 

 Mendelian transmission is all the more remarkable in that the 

 characters are not 'superficial but deep-seated and relating-to bodily 

 conformation, each of them depending on a number of anatomical 

 factors. It cannot be said, however, that all the characters were 

 inherited " pure " or without being influenced by the other charac- 

 ters with which they entered into new combinations, and in some 

 cases the points shown were definitely composite, being dissimilar 

 to those of both the parent breeds. 



When, as a consequence of cross-breeding two varieties, the 

 alteration of the parent characteristics is minimal the transmission 

 of pure characters — ^^according to Mendelian expectation— -may be 

 said to occur, and experimental evidence has shown that there are 

 considerable numbers of such cases. It is a legitimate field of 

 work for the biometrical school of biology to determine by 

 statistical methods the extent to which variation occurs as a 

 result of attempted superposition of characters which in their 

 "pure" state are physiologically incompatible. Furthermore, a 

 latent character may be regarded as one, the outward manifesta- 

 tion of which is incompatible with the existing kind of metabolism, 

 but which is capable of reappearance as soon as the conditions 

 become favourable. But because it is helpful to assume that latent 

 characters are present in some manner in the animal organisation, it 

 is not necessary to assume that they are definitely located in the nuclei 

 of germ-cells or in any other particular structures ^ (see pp. 674-675). 



Moreover, it should be remembered that there is no experimental 

 proof that the chromosomes of the gametes constitute the entire 

 physical basis of inheritance. The best evidence in support of this 

 supposition appears to be Boveri's experiment, in which he fertilised 

 a non-nucleated ovum of one species of sea-urchin with the sper- 

 matozoon of another species.^ The resulting pluteus or larva was 

 purely paternal in its characters. Boveri concluded, therefore, that 

 this result was due to the introduced nucleus, the maternal cytoplasm 

 having no determining effect upon the offspring, but merely supplying 



' The attempt to locate latent characters of organisms in particular parts of 

 the germ-cells should perhaps be regarded as a survival from a time when all 

 kinds of qualities, abstract or otherwise, were supposed to reside in deiSnitely 

 restricted positions. Compare Phenology. The centres in the nervous system 

 are not comparable, since these are to be regarded as parts of mechanisms foi' 

 controlling different functions. The centres preside over the respective func- 

 tions, but the functions themselves are not located in the centres. 



2 Boveri, "Ein Geschlechtlich erzeugter Organismus ohne Mutterliche Eigen- 

 schaften," S. B. d. Ges. f. Mcrph. w. Phys., Miinchen, vol. v., 1889. 



