PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 353 



('iiusatioii of detinite anomalies iu the lu-tiis is laruely ;i matter of in- 

 dividual opiuion. 



It is denied by some lii^uii autliorities. led by Beisi'man and Leuek- 

 Lart.* ]\rost praetitiouers, however, believe in it, and I need hardly 

 add that it is a universal. |>oi)ul;ir bi'lief, t supported by uumerous 

 cases. 1 myself am a firm believer iu it. The beariug which the sub- 

 ject has upon this discussion is this: If a dcAiation in the develoj)- 

 meut of a child is produced by matermil impression, we have a proof 

 that a deviatiou from uormal hereditary teudeucies can be ])roduced 

 without either direct vascular or nervous continuity. 



We see an analogy between the experiments of Brown-8e(fuard, the 

 influence of the i)revious sire, and the maternal influence. Neither, in 

 mj' opinion, directly supi)orts the theory of transndssion of ac(piired 

 characters, for they do not prove that normal chan.yes in the body cells 

 directly react upcni the ,nerm cells; they all show that the typical 

 hereditary development of single organs m;iy be diverted by living- 

 forces which have no direct connection with them according to our 

 present knowledge. 



AYhat the nature of these forces is 1 will not undertake to say, but 

 I believe we must adnut the existence of some unknown force, or rather 

 of some unknown relati«ms between the body cells and germ cells. 



A j^ear ago, recognizing fully tlie difliculty of advancing any theory 

 of heredity which would exi)lain the transmission of acquired characters, 

 I came to the following result : " It follows as an unprejudiced conclusion 

 from our present evidence that upon Weismann's princiide we can 

 ex])lain inheritance but not evolution, while witli Lamarck's principle 

 and Darwin's selection jninciple we can explain evolution, but not, at 

 present, inheritance. Disprove Lamarck's princi])le andwenuist assume 

 that there is some third factor in evolution of which we are now 

 ignorant."' 



In this connection it is interesting to (juoteagain from my colleague, 

 Prof. E. n. Wilson. lie writes that the tendency in Germ.any at 

 lU'csent is to turn from speculation to empiricism, and this is due 

 l>artly "• to the feeling that the recent wondeiiul ad\ances in our 

 knowledge of cell phenomena Inne enormonsly increased t he difliculti«'s 

 of a purely meciianico physical exi)lanation of vital phenomena. In 

 fact, it seems that the tendem-y is to turn back in the direction of the 

 vital-force conception. - - - As J3uveri said to me recently, "Es 

 gibt zu viel vorstand in der Natur um cine rein mechanische Erkliirnng 

 del' Sache zu ernniglichen." 



In tlie linal lecture we turn to the forces exhibited in tlie germ cells. 



H((nd('nrilrterhit<lt der I'hjiaioUxjie, Wajrnor, Artikcl " Zeuguiij;-,'' Leuckhiirt. 

 tSec, Medical Hvci>rd, OctolxirlM, 1891, iiii article by .Juscpli Drzt'wiecrki, m. d. 



H. Mis. 114 L'.'} 



