338 Report S.A.A. Advancement of Science. 



zygote; another fact which points to the extreme importance of 

 the nucleus. Thus the developing organism comes to be controlled 

 by stimuli which are derived from two parents and which will differ 

 according to the nature of the environment to which those parents 

 and their ancestors have been exposed. 



Two separate store-houses of ancestral stimuli, and therefore 

 two separate centres of control, meet together in the zygote, and 

 there are various possibilities as tO' the manner in which they will 

 share between them the work of controlling, by progressive equilibra- 

 tion, the developing organism : — 



(i) The male and female centres of control may be able to blend 

 their forces intimately in the so-called segmentation nucleus, and 

 the developing organism will then be controlled by the resultant, 

 and may be expected to exhibit characters more or less 

 intermediate between those of the two parents. This appears 

 to be the most usual condition of things and it is tO' such cases 

 that Galton's law of inheritance applies, viz. : " That the twq 

 parents contribute between them on the average one-half, or 

 (o"5) of the total heritage of the offspring; the four grand- 

 parents, one-quarter, or (o"5)-; the eight great-grandparents, one 

 eighth, or (o"5)-^ and so on. Then the sum of the ancestral 

 contributions is expressed by the series 



|(o•5) + (o•5)- + (o•5)^&c. |, 



which, being equal to i, accounts for the w^hole heritage." 



(2) The male and female controlling centres may be incapable of 

 blending their forces ; in this case they may separate completely 

 at the first or at some subsequent division of the segmentation 

 nucleus, and thereafter each may control a certain fraction of 

 the developing organism, yielding a lopsided result. This is 

 probably the explanation of those remarkable abnormal insects 

 mentioned by Darwin, in which one half or one quarter of the 

 body is like that of the male and the other half or three quarters 

 like that of the female.'^ Such cases are, however, extremely 

 rare and only occur by way of abnormality. 



(3) The male and female centres of control may remain for one 

 or more generations associated in the same nuclei (though one 

 may completely dominate over the other in its influence upon 

 the developing organism) ; but they become dissociated sooner 

 or later on the formation of new germ-cells (gametes), the 

 gametes bearing the different types of nuclei respectively being 

 produced on an average in equal numbers. To' this category 

 must be referred those cases described by Mendel and Bateson 

 in which the offspring of a cross graduallv separate themselves 

 out in definite numerical proportions. 



In these latter cases we must suppose that there is in the first 

 instance only an incomplete amalgamation of the male and female 

 pronuclei, the differential characters being sufiicient to prevent perfect 



* Animals and Plants under Domestication, Vol. II., p. 394. 



