No. 494] ZOOLOGICAL PROGRESS 



through that body conforms in large measure to prin- 

 ciples already discovered in the physical and chem- 

 ical laboratories, it has been generally assumed that the 

 life processes of an animal are nothing more than com- 

 plex examples of physico-chemical interaction. This idea 

 has proved most stimulating in its effect on research, but 

 to what extent it will be found true can not at present be 

 stated. It is quite possible that the chemist and physicist 

 have as much of a fundamental nature to learn from living 

 material as they have already gained from lifeless sub- 

 stance. 



The paramount influence of material in animal reac- 

 tions can not better be illustrated than in such processes 

 as inheritance. It is well known to you how much more 

 closely on the average offspring resemble their parents 

 than they do other members of their own species, and 

 you are familiar with the long persistence of certain 

 family traits. These resemblances are explained by the 

 fact that the offspring start from a certain amount of 

 living substance contributed by each parent, but the pow- 

 erfully determinative qualities of this substance are only 

 to be appreciated in certain cases. It is well known that 

 in human beings there are two classes of twins, identical 

 twins and ordinary twins. The latter come from two sep- 

 arate eggs and may vary as much from each other as any 

 two members of the same family. The former come from 

 a single egg which by some accident has become separated 

 into two parts. Identical twins are always of the same 

 sex and are often so alike in features and actions that 

 they are almost indistinguishable even to their near asso- 

 ciates. Their close similarity, which may amount almost 

 to identity, shows that the substance from which they 

 both came has developed in a most rigidly uniform way 

 and indicates that the development of ordinary twins, as 

 well as of other individuals, is probably also closely lim- 

 ited from the beginning, but the degree of this limitation 

 is not discoverable in these cases, for we have no basis 

 of comparison. Since living material can thus duplicate 



