234 BIOLOGY: GENERAL AND MEDICAL 



it, that some kind of physiological units are engaged in 

 the phenomena of heredity and centres them in the germ 

 plasm which is believed to be continuous from genera- 

 tion to generation, it progresses much further and pro- 

 poses to show the source of the germ plasm and the exact 

 location, distribution, and treatment of the units. 



As Weismann's theory is of such importance, it will be 

 dwelt upon at some length and as nearly as possible be 

 given in the author's own language. 



" All the phenomena of heredity depend upon minute vital units 

 which we have called biophors and of which living matter is 

 composed: these are capable of assimilation, growth, and multipli- 

 cation by division. We are unacquainted with the lowest con- 

 ceivable organisms, and do not even know if they still exist. 

 But they must at any rate have done so at some time or other, 

 in the form of single biophors, in which multiplication and trans- 

 mission occurred together, no special mechanism for the purpose 

 of heredity being present. When the body became constructed in 

 a more or less complex manner, of various kinds of biophors 

 arranged in a definite manner, simple binary fission no longer 

 sufficed for the transmission of the characters of the parent to the 

 offspring. If the parts situated in the anterior, posterior, right, 

 left, dorsal,*and ventral regions differed from one another, all the 

 elements i.e., all the kinds and groups of biophors could not 

 by any method of halving, be transmitted to both the offspring 

 resembling the parent. Special means must then have been 

 adopted to render such a completion and consequent perfect 

 transmission possible; and this was attained by the formation of a 

 nucleus. We may regard the nucleus as having originally served 

 merely for the storage of reserve biophors. Subsequently that 

 is, in multicellular organs possessing highly differentiated cells 

 the nucleus took on other functions, which regulated the specific 

 activity of the cell, though it still retained biophors capable of 

 supplying the characters of the cells which were still wanting and 

 therefore still better served as the bearer of the biophors con- 

 trolling the character of the cell. If, therefore, a special apparatus 

 for transmission became necessary in the hetero-biophorids or 

 unicellular organisms and appeared in the 'cell ' in the f orin of a 

 'nucleus,' it must have become still more complex on the intro- 

 duction of the remarkable process of amphimixis, which, in its 

 simplest and original form, consists in the complete fusion of two 

 organisms in such manner that nucleus unites with nucleus and 

 cell body with cell body (conjugation). In the higher unicellular 

 organisms this process is, in most cases, restricted to the fusion of 

 the nuclei half the nucleus of one animal uniting with half that of 

 another. The process of division shows that the nucleus has a 

 structure precisely analogous to that of the nucleus in multicellular 

 organisms; we may, therefore, assume that the hereditary sub- 



