CRITICISM 65 



the bearers of hereditary characters, we should expect some degree 

 of correspondence between the differences distinguishing the types 

 and the visible differences of number or shape distinguishing the 

 chromosomes. Moreover, if the chromosomes were the chief gov- 

 ernors of structure we should expect to find greater differences 

 between them in different tissues of the same body. 



6. No one has protested more clearly and vigorously than Guyer 

 (1909, 1911) against "the inordinate importance which has been 

 attributed to the chromosomes as vehicles of heredity." He points 

 out, for instance, that there is definite experimental evidence of 

 the great importance of the ovum-cytoplasm, and argues that " the 

 number and arrangement of the chromosomes in a given species are 

 the effects of the fundamental constitution of a given kind of living 

 matter, rather than that they stand in a specifically causal relation 

 to such constitution." " Heredity is the problem of the handing-on 

 of metabolic energies already established, rather than of the trans- 

 mission of a series of determinative units which create a wholly new 

 organism." " This much is certain ; no chemical, physiological, or 

 morphological evidence is yet extant which places the hereditary 

 factors wholly within the chromosomes." It seem highly probable 

 that the chromosomes " control the velocities in cell-chemistry " by 

 supplying the proper amounts and kinds of ferments which act on 

 a series of fundamental cell-constituents that are largely common 

 to both egg and sperm. 



Perhaps then the safest conclusion at present is that the chromo- 

 somes, along with other germ-cell constituents, " stand in some definite 

 causal relation to adult characters." 



In T. H. Morgan's Physical Basis of Heredity (1919) a convinced 

 statement will be found of the view that the main vehicles of the 

 hereditary qualities are the chromosomes of the nuclei of the germ- 

 cells. These chromosomes contain the differentiated areas (genes 

 or factors) which are causally connected with the development of 

 certain characters. But the cytoplasm of the egg-cell may contain 

 hereditary plastids and building materials of importance ; and 

 in any case a germ-cell is a unity like a firm. 



