[macallum] semi-centennial OF THE ORIGIN OF SPECIES 189 



affect that part in the offspring except in so far as use or disuse may 

 affect the nutrition of the whole body. 



That the nuclear membrane has extraordinary selective powers^, as 

 regards the substances it permits to diffuse throught it, may be gathered 

 from a few facts. Never, except in pathological tissues, does the nucleus 

 contain fats, carbohydrates, or salts, even when the cytoplasm is charged 

 with these. Further the cytoplasm abounds in proteins but these, if 

 they occur at all, are only in traces in the nuclear cavity. The mem- 

 brane is the baiTier to their entrance. It, however, permits iron-holding 

 nucleo-proteins to diffuse through it. This is seen in the pancreatic 

 cells whose nuclei give origin to an iron-holding nucleo-protein, which I 

 denominated prozymogen, found in the cytoplasm. Further the Nissl 

 granule material in the cytoplasm of the nerve cell of adult vertebrates 

 reaches the cytoplasm by diffusion from the nucleus. 



The nucleus, on this conception of its functions, owed its origin to 

 the fact that some intracellular organ was required to receive the here- 

 dity-bearing substance and protect it from change chemical and physical. 



Before a nucleus arose heredity must have been a very uncertain 

 factor for what then corresponded to chromatin must have been sub- 

 jected to the action of everything that invaded the cytoplasm from with- 

 out and especially when the salts of the Archean Ocean increased in 

 concentration. Indeed it was probably the latter that constituted the 

 chief factor in the evolution of the nucleus. There are still, it is true, 

 organisms which are without a nucleus; the Blue-green Algag, Bacteria 

 and Yeasts. These have external membranes which in a measure protect 

 them from the action of the salts of their environment, and that is, per- 

 haps, the reason why they only have survived from a time when all 

 organisms were non-nucleated. 



The nuclei then are the receptacles for all the iron-holding nucleo- 

 proteins formed in the cells of the organism. What kind of such com- 

 pounds is stored up in a nucleus depends on the physical and chemical 

 properties of its membrane. In the nuclei of the germ-cells a nuclear 

 membrane of a certain quality is present. In the somatic nuclei mem- 

 branes of other qualities must obtain. To the germ-cells must, on this 

 basis, go one kind of chromatin not absolutely and always the same. 

 Slight alterations in its composition may not prevent it reaching the 

 nuclei of the germ-cells and may thus provide for variation. The 

 Bomatic cells under certain conditions of environment and of nutrition 

 may constantly prepare such slightly altered chromatin, and thus a re- 

 gular tendency to variation may obtain. In this way also use and disuse 

 of an organ in an animal can, after a time, influence its offspring. 



