ADDRESS, 27 



of the aorta ; in birds the corresponding part of the aorta is an enlarge- 

 ment of the fourth right arch, and in reptiles both arches persist to form 

 the great artery. That this original symmetry exists also in man we 

 know from the fact that now and again his body, instead of correspond- 

 ing with the mammalian type, has an aortic arch like that which is 

 natural to the bird, and in rarer cases even to the reptile. A type form 

 common to the vertebrata does therefore in sucli cases exist, capable of 

 evolution in more than one direction. 



The reputation of Thomas Henry Huxley as a philosophic compara- 

 tive anatomist rests largely on his eai'ly perception of, 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 prin ciple 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 characters. The transmission of these charac- 

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

 possessed by the chromosome constituents of the segmentation nucleus in 

 the fertilised ovum, named by him the germ plasm, which is continued 

 from one generation to another, and impresses 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 differentiated in structure and function, and 

 so long as the differentiation follows certain prescribed lines the morpho- 

 logical characters of the species are preserved. We can readily conceive 

 that, as the process of specialisation is going on, modifications 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 assume the arrangements found in 

 another species, or even in another order. Anatomists had indeed long 

 recognised that variations from the customary ari-angement of parts 

 occasionally 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 apprehended until a flood of light was thrown on the entire 



