394 



SCIENCE. 



[N. S. Vol. XII. No. 298. 



and insistence on, the necessity of testing 

 morphological conclusions by a reference 

 to the development of parts and organs, 

 and by applying this principle in his own 

 investigations. The principle is now so 

 generally accepted by both botanists and 

 anatomists that morphological definitions 

 are regarded as depending essentially on 

 the successive phases of the development 

 of the parts under consideration. 



The morphological characters exhibited 

 by a plant or animal tend to be hereditarily 

 transmitted from parents to offspring, and 

 the species is perpetuated. In each species 

 the evolution of an individual, through the 

 developmental changes in the egg, follows 

 the same lines in all the individuals of the 

 same species, which possess therefore in 

 common the features called specific char- 

 acters. The transmission of these charac- 

 ters is due, according to the theory of Weis- 

 mann, to certain properties possessed by 

 the chromosome constituents of the segmen- 

 tation nucleus in the fertilized ovum, named 

 by him the germ plasm, which is continued 

 from one generation to another, and im- 

 presses its specific character on the egg 

 and on the plant or animal developed 

 from it. 



As has already been stated, the special 

 tissues which build up the bodies of the 

 more complex organisms are evolved out of 

 cells which are at first simple in form and 

 appearance. During the evolution of the 

 individual, cells become modified or differ- 

 entiated in structure and function, and so 

 long as the differentiation follows certain 

 prescribed lines the morphological charac- 

 ters of the species are preserved. "We can 

 readily conceive that, as the process of spec- 

 ialization is going on, modifications or var- 

 iations in groups of cells and the tissues 

 derived from them, notwithstanding the 

 influence of heredity, may in an individual 

 diverge so far from that which is character- 

 istic of the species as to assume the ar- 



rangements found in another species, or 

 even in another order. Anatomists had in- 

 deed long recognized that variations from 

 the customary arrangement of parts occas- 

 ionally appeared, and they described such 

 deviations from the current descriptions as 

 irregularities. 



DARWINIAN THEORY. 



The signification of the variations which 

 arise in plants and animals had not been ap- 

 prehended until a flood of light was thrown 

 on the entire subject by the genius of 

 Charles Darwin, who formulated the wide- 

 reaching theory that variations could be 

 transmitted by heredity to younger gener- 

 ations. In this manner 'ke conceived new 

 characters would arise, accumulate, and be 

 perpetuated, which would in the course of 

 time assume specific importance. New spe- 

 cies might thus be evolved out of organisms 

 originally distinct from them, and their spe- 

 cific characters would in turn be trans- 

 mitted to their descendants. By a contin- 

 uance of this process new species would 

 multiply in many directions, until at length 

 from one or more originally simple forms 

 the earth would become peopled by the in- 

 finite varieties of plant and animal organ- 

 isms which have in past ages inhabited, or 

 do at present inhabit, our globe. The Dar- 

 winian theory may therefore be defined as 

 Heredity modified and influenced by Vari- 

 ability. It assumes that there is an hered- 

 itary quality in the egg which, if we take the 

 common fowl for an example, shall continue 

 to produce similar fowls. Under condi- 

 tions, of which we are ignorant, which oc- 

 casion molecular changes in the cells and 

 tissues of the developing egg, variations 

 might arise, in the first instance probably 

 slight, but becoming intensified in succes- 

 sive generations, until at length the de- 

 scendants would have lost the characters 

 of the fowl and have become another 

 species. N'o precise estimate has been ar- 



