546 ON HEREDITY. 



place.* It is obvious therefore that the molecules which enter iuto the 

 forination of the segmeutation nucleus can move within its substance, 

 and can undergo a re-adjustment in size and form and position. But 

 this re-adjustment of material is without doubt not limited to those 

 relatively coarse particles which can be seen and examined under the 

 microscope, but applies to the entire molecular structure of the segmen- 

 tation nucleus. Now it must be remembered that the cells of the em- 

 bryo from which all the tissues and organs of the adult body are de- 

 rived are themselves descendants of the segmentation nucleus, and they 

 will doubtless inherit from it both the power of transmitting definite 

 characters and a certain capacity for re-adjiistraent both of their con- 

 stituent materials and the relative positions which tliey may assume 

 towards each other. One might conceive therefore that if in a suc- 

 cession of organisms derived from common ancestors the molecular 

 particles were to be of the same composition and to arrange themselves 

 in the segmentation nucleus and in the cells derived from it on the 

 same lines, these successive generations would be alike ; but if the lines 

 of adjustment and the molecular constitution were to vary in the differ- 

 ent generations, then the products would not be quite the same. Vari- 

 ations in structure and to some extent also in the construction of parts, 

 would arise, and the unlike would be produced. 



In this connection it is also to be kept in mind that in the higher organ- 

 isms, and indeed in multicellular organisms generally, an individual is 

 derived, not from one parent only, but from two ])arents. If one parent 

 were to contribute a larger proportion than the other to the formation of a 

 particular organism, then the balance would be disturbed, the offspring in 

 its character would incline more to one parent than to the other, accord- 

 ing to the proportion contributed by each, and a greater scope for the 

 production of variations would be provided. These differences would be 

 increased in number in the course of generations, owing to new combi- 

 nations of individual characters arising in each generation. As long 

 as the variations which are produced in an organism are collectively 

 within a certain limitation, they are merely individual variations, and 

 express the range within which such an organism, though exhibiting 

 differences from its neighbors, may yet be classed along with them in 

 the same species. It is in this sense that I have discussed the term 

 variability up to the present stage of this address. Thus all those 

 varieties of mankind which on account of differences in the color of 

 the skin, we speak of as the white, black, yellow races and red-skins 

 are men, and they all belong to that species which the zoologists term 

 Homo sapiens. 



But the subject of variability cannot, in the pi-esent state of science, 

 be confined in its discussion to the production of individual variations 



*The observations, inoio especially of Flemming, E. Van Beudeueu, Strasburger, 

 and Cainoy, may be referred to in connection with the changes which take place in 

 nuclei prior to, and in connection with, their division. 



