334 PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 



state most clearly what is and what is uot involved in this discussion. 

 Weismann* does not claim that the reproduction or jicrm cells areun- 

 iuduenced by habit; on the other hand, he admits that most important 

 modifications in these cells may and do result from changes of food, 

 climate, from healthy or unhealthy conditions of the body; also from 

 infectious disease, where it is quite as possible that the microbes may 

 enter the reproductive cells as any other cells of the body; from alco- 

 holism, where the normal molecular acticm of the protoplasm of the 

 germ cells may be disturbed, resulting in abnormal development, and 

 there are some very interesting experiments which 1 shall cite on this 

 j)oint; from some nervous disorders which profoundly modify cell- 

 function in all the tissues: in other words, ornni mnum in corpore sano. 

 But to accept all this, and even to iiichuh^ all our rapidly increasing 

 knowledge of the direct relation between such phenomena as produc- 

 tion of deformities and determination of sex, and the influences of 

 environment u]>on the ovum; or the inliueiu'es of the mother upon the 

 fcetus — this is all aside from the real question at issue. 



It may be stated thus: Given G, the ova and si)ermatozoa, the germ 

 cells or material vehicles of hereditary characters; A', the body or so- 

 matic; cells of all the other tissues conveying the hereditary characters 

 of nerve, muscle, and bone; F, the variations in these body cells "ac- 

 quired" during lifetime; given these factors, the real question is: Do 

 influences at work producing variations in certain body cells of the 

 parent so alfect the germ cells of the i)arent that they re-appear in cor- 

 responding body cells of the offs])ring ? To take a concrete case, will 

 the increased use of the cells of the extensor iiulicis nuiscle in the 

 parent so stimulate that portion of the germ cells Avhich represents this 

 muscle that the increment of growth will in any degree re-ai)})ear in 

 the offspring ? 



This is what is reipiired of heredity upon the Lamarckian hypothe- 

 sis, and r think you will see at once that while this hypothesis sim- 

 plifies the problem of evolution it in a corresponding degree renders 

 more difficult the problem of heredity — for we have uot the first ray of 

 knoM'ledge of what such a process involves. There is no quality more 

 essential to scientific progress than common honesty; if we take a 

 position let us face all its consequences; the more we reflect upon it 

 the more serious the Lamarckian position becomes. 



In the present lecture let us first briefly review the progress of the 

 science of heredity which has led up to the present discussion. Second, 

 let us examine the e\'idence for and against the Lamarckian theory, 

 and inquire how far natural selection can explain all the facts of evolu- 

 tion. Third, let us examine the evidence for such a c(uitinuous relation 

 between the body cells and germ cells as must exist if the Lamarckian 

 theory is the true one. 



History of the heredity theory. — In a \aliiable summary of the past 



* See Essay.s iij)ou Heredity aud Kindred liiological Problems, 1889. Trans. 



