ADDRESS BEFORE THE BRITISH ASSOCIATION. 45 



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 characters. The transmission of these char- 

 acters is due, according to the theory of Weismann, to certain properties 

 possessed by the chromosome constituents of the segmentation nucleus 

 in the fertilized ovum, named by him the germ plasm, which is con- 

 tinued from one generation to another, and impresses its specific char- 

 acter 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 differentiated in structure and func- 

 tion, and so long as the differentiation follows certain prescribed lines 

 the morphological characters of the species are preserved. We can 

 readily conceive that, as the process of specialization is going on, modi- 

 fications or variations 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 characteristic of the species as to as- 

 sume the arrangements found in another species, or even in another 

 order. Anatomists had, indeed, long recognized that variations from 

 the customary arrangement of parts occasionally appeared, and they de- 

 scribed such deviations from the current descriptions as irregularities. 



DARWINIAN THEORY. 



The signification of the variations which arise in plants and animals 

 had not been apprehended 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 generations. In this manner he conceived new characters 

 would arise, accumulate and be perpetuated, which would in the course 

 of time assume specific importance. New species might thus be evolved 

 out of organisms originally distinct from them, and their specific char- 

 acters would in turn be transmitted to their descendants. By a con- 

 tinuance 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 infinite varieties of plant and animal 

 organisms which have in past ages inhabited, or do at present inhabit 

 our globe. The Darwinian theory may, therefore, be defined as 

 heredity modified and influenced by variability. It assumes that there 



