46 POPULAR SCIENCE MONTHLY. 



is an heredity quality in the egg, which, if we take the common fowl for 

 an example, shall continue to produce similar fowls. Under conditions, 

 of which we are ignorant, which occasion molecular changes in the cells 

 and tissues of the developing egg, variations might arise in the first in- 

 stance probably slight, but becoming intensified in successive genera- 

 tions, until at length the descendants would have lost the characters of 

 the fowl and have become another species. No precise estimate has 

 been arrived at, and, indeed, one does not see how it is possible to obtain 

 it, of the length of years which might be required to convert a variation, 

 capable of being transmitted, into a new and definite specific character. 



The circumstances which, according to the Darwinian theory, deter- 

 mined the perpetuation by hereditary transmission of a variety and its 

 assumption of a specific character depended, it was argued, on whether 

 it possessed such properties as enabled the plant or animal in which it 

 appeared to adapt itself more readily to its environment, i. e., to the 

 surrounding conditions. If it were to be of use, the organism in so far 

 became better adapted to hold its own in the struggle for existence with 

 its fellows and with the forces of nature operating on it. Through 

 the accumulation of useful characters the specific variety was perpetu- 

 ated by natural selection so long as the conditions were favorable for its 

 existence, and it survived as being the best fitted to live. In the study 

 of the transmission of variations which may arise in the course of devel- 

 opment, it should not be too exclusively thought that only those varia- 

 tions are likely to be preserved which can be of service during the life of 

 the individual, or in the perpetuation of the species, and possibly avail- 

 able for the evolution of new species. It should also be kept in mind 

 that morphological characters can be transmitted by hereditary descent, 

 which, though doubtless of service in some bygone ancestor, are in 

 the new conditions of life of the species of no physiological value. Our 

 knowledge of the structural and functional modifications to be found 

 in the human body, in connection with abnormalities and with tend- 

 encies or predisposition to diseases of various kinds, teaches us that 

 characters which are of no use, and indeed detrimental to the individual, 

 may be hereditarily transmitted from parents to offspring through a suc- 

 cession of generations. 



Since the conception of the possibility of the evolution of new 

 species from pre-existing forms took possession of the minds of natural- 

 ists, attempts have been made to trace out the line on which it has 

 proceeded. The first to give a systematic account of what he conceived 

 to be the order of succession in the evolution of animals was Ernst 

 Haeckel, of Jena, in a well-known treatise. Memoirs on special depart- 

 ments of the subject, too numerous to particularize, have subsequently 

 appeared. The problem has been attacked along two different lines: 

 the one by embryologists, of whom may be named Kowalewsky, Gegen- 



