354 COOK 



possible to suppose that if the granules derived from a given 

 ancestor secure a favorable position the characters of that ances- 

 tor will predominate in the new individual. In this way the 

 characters of different ancestors might assert themselves in end- 

 lessly varied degrees, even in the offspring of the same parents, 

 as they often do. This theory has the advantage of affording 

 a thinkable connection between facts which otherwise appear 

 completely mysterious. Two collateral circumstances increase 

 the warrant for applying the suggestion to the phenomena of 

 heterism. 



It has been indicated by several observers, but most directly 

 by Prowazek l that the granules of chromatin, which compose 

 the chromosomes at the period of the conjugation, migrate, dur- 

 ing vegetative growth, to positions at the knots of the nuclear 

 network, as though to direct the processes of assimilation and 

 growth. It was found by Maupas in his experiments with 

 infusoria that continual inbreeding causes the gradual deterior- 

 ation and diminution of the nucleus, as though diversity of 

 descent were necessary to maintain the nuclear network, either 

 by keeping up the number of granules or by enabling them to 

 stay at the right distance apart. Such a relation would explain 

 the known facts, to the extent of indicating a reason for heterism 

 and a means for bringing it about. 2 



It is also easier to conceive of the possibility of bud-variations 

 under the supposition that the influences exerted by the chrom- 

 atin depend upon position, rather than upon the origination of 

 new units or upon the making of different combinations. Modi- 

 fications of hereditary forms and methods of growth do occur 

 during the vegetative period, as already stated, and may be 

 quite as pronounced as the mutations obtained from seed. 

 Changes capable of accounting for bud-variations would also 

 be adequate for the explanation of mutative variations. 



Those who begin with the assumption that evolutionary prog- 

 ress is actuated by external causes are compelled to argue that 

 the diversities of individual organisms arise through varied 



1 Prowazek, J., 1904. Keimveranderungen in Myxomycetenplasmodium. Oes- 

 terreich. Bot. Zeitsch., 54: 27S. 



2 Cook, O. F. and Swingle, W. T., 1905. Evolution of Cellular Structures. 

 Bui. 8i, Bureau of Plant Industry, U. S. Dept. of Agriculture. 



