44 POPULAR SCIENCE MONTHLY. 



archetype vertebrate form which had no existence in nature, and to 

 which, subject to modifications in various directions, he considered all 

 vertebrate skeletons might be referred. Owen's observations were con- 

 ducted to a large extent on the skeletons of adult animals, of the knowl- 

 edge of which he was a master. As in the course of development modi- 

 fications in shape and in the relative position of parts not unfrequently 

 occur, and their original character and place of origin become obscured, 

 it is difficult, from the study only of adults, to arrive at a correct inter- 

 pretation of their morphological significance. When the changes which 

 take place in the skull during its development, as worked out by 

 Reichert and Eathke, became known and their value had become ap- 

 preciated, many of the conclusions arrived at by Owen were challenged 

 and ceased to be accepted. It is, however, due to that eminent 

 anatomist to state from my personal knowledge of the condition 

 of anatomical science in this country fifty years ago, that an enormous 

 impulse was given to the study of comparative morphology by his writ- 

 ings, and by the criticisms to which they were subjected. 



There can be no doubt that generalized arrangements do exist in the 

 early embryo which, up to a certain stage, are common to animals that 

 in their adult condition present diverse characters, and out of which the 

 forms special to different groups are evolved. As an illustration of this 

 principle, I may refer to the stages of development of the great arteries 

 in the bodies of vertebrate animals. Originally, as the observations of 

 Rathke have taught us, the main arteries are represented by pairs of 

 symmetrically arranged vascular arches, some of which enlarge and con- 

 stitute the permanent arteries in the adult, whilst others disappear. The 

 increase in size of some of these arches, and the atrophy of others, are so 

 constant for different groups that they constitute anatomical features 

 as distinctive as the modifications in the skeleton itself. Thus in mam- 

 mals the fourth vascular arch on the left side persists, and forms the 

 arch of the aorta; in birds the corresponding part of the aorta is an en- 

 largement 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 corre- 

 sponding 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 such 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 early 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 principle is now so generally accepted by both 

 botanists and anatomists that morphological definitions are regarded as 



