PRESENT PROBLEMS IN EVOLUTION AND IIEHEDITV. 373 



we should tirst tlioiouulilN' uiuU'istaml the t'ssnitial plieuoineiia of 

 normal life. Of course we shall never see life as it really is, because 

 there is always something beyoiul our highest magnifying ])owers; but 

 we come nearest to this invisible form of energy when, with such 

 iiivestigat(n's as Hertwig and IVIaui)as, we strip the life processes of all 

 theii' accessories and view them in their simplest external form. 



The problems of evolution are found to be inseparably connected with 

 those of heredity. No theory is at all adequate which does not ex])lain 

 both classes of facts, and we have seen tliat the explanations offered 

 by tlu' two oi)posed schools — those who l)elieve in the transmission of 

 acquired chara<'lers and those who do not — are directly exclusive of 

 each other. We should suspend Judgment entirely rather than cease 

 to gather from every quarter facts wliich bear upon the most important 

 and central i)roblem of the transmission of acquired characters. 1 

 have endeavored to ])oiut out the opportunities which medical practi- 

 tioiu'rs enjoy of contributing evidence upon tliis mooted (juestion. It 

 must not be forgotten that while the inheritance of individual adapta- 

 tion to environment is the simplest method of e\i>iaining race 

 adaptation such as we observe in the evolution of num, we know abso- 

 Intely nothing of how snch inheritance can be effected through tlu^. 

 germ cells. We can not at present construct even any form of working 

 hypothesis for such a ]>rocess. On the other hand, we have found how 

 untenable is the alteiuative theory offered to us by Weismann, that it 

 is solely natural selection or the survival of the Attest which 



." - - - sli;qics oni- fiids, 

 l\()iij;li how tliciii as w <■ will." 



At the same time Weismann's concei)tion of it continuity of germinal 

 protoplasm, which we ha\-e fouiul to consist in chromatin i)lus archo- 

 plasm, helps us over many of the phenomena of heredity, especially on 

 the retrogressive side, and if it were not that we nuist also account 

 for progressive and detiiiite transformation in heredity, we miglit credit 

 the distinguished Freiburg natui alist with having loosened the Gor- 

 dian knot. 



In summing u|>, the order of treatment (bllowed in the lectui'cs may 

 be reversed, and we can begin with the germ cells, and condense the 

 more or less ascertaincil facts. 



The germ cells: 



(1) The material substance of hereditary transmission is the liighly 

 coloring proto]>lasm. or chromatin, in the nucleus ol' the germ cells, 

 probably connected with a certain form of archoplasm. or dynamic 

 protoplasm outside of the nucleus. 



(!') Before conjugation and i'ertilization the hereditary substance 

 of both the male and femide cells is leduced to one-half tliat found in a 

 typh-al cell. This substance is iu)wever hrst doubled and tlien (juar- 

 tered, the meaning of which process is not understood. 



