niche, but a continual adjustment to changing niches. This 

 theory can account better for the sequence and replacements 

 of faunas known to have lived in any given area. It differs 

 from the former concept only in its explanation of method 

 and perhaps in its starting point. 



The theory of evolution, based on natural selection 

 (Darwin, 1858, 1859),. did not materially change the direc- 

 tion of study in comparative morphology. With the general 

 acceptance of this modus operandi of life, comparative mor- 

 phologists switched easily from the documentation of the 

 numerous and minutely different steps in a chain, or chains 

 of being, to the study of the phylogeny of the various evolu- 

 tionary lines. 



At this time the biogenetic law (Serres, Haeckel) was being 

 formulated. This law asserted that the embryological stages 

 an organism passed through in its development resembled 

 the adult stages of its ancestry. Haeckel stated this succinctly 

 as "Ontogeny recapitulates phylogeny." A general accept- 

 ance of this view made embryology an important part of 

 comparative morphological research. Unfortunately the 

 equating of ontogeny and phylogeny introduced a great deal 

 of confusion as well. 



The biogenetic law, like the theory of evolution, was not 

 accepted by everyone and there was considerable debate 

 about its shortcomings and strong points. It was denied from 

 the beginning by some (von Baer), and others sought to 

 discredit it primarily because of its choice of words. Argu- 

 mentation about this concept has continued until the present, 

 although there is general agreement that it is not a law. 



On the one hand, Aulie (1955) affirms that "Recapitula- 

 tion . . . has turned out to be something quite unlike what 

 it was first thought to be. It is not the mystical expression 

 of some creative force in nature as the Nature-Philosophers 

 lead by Kielmeyer had thought; nor the mechanical pushing 

 back of characters, as Weismann would have it; neither is 

 it a simple expression of heredity, as Haeckel had obscurely 

 maintained The phenomena of recapitulation cannot be 

 deduced from some simple general law, but rather they must 

 be viewed as occurring in an organism developing in the 

 most efficient manner possible, each stage of which is placed 

 with careful reference to future development, as an individ- 

 ual expression of relative growth, and the fitting in of genetic 

 actions into the life of the individual." Needham (1930) 

 also states: "It is the function of the embryo to become an 

 adult without looking backward on ancestral history." In 

 short, recapitulation is only partial and open to short cuts 

 or additions at any stage, as pointed out by von Baer. 



On the other hand, Hyman (1940) asserts that "Resem- 

 blance to ancestral forms, embryonic or adult, is a common 

 and widespread phenomenon . . . consequently the practice 

 of drawing phylogenetic conclusions from the study of de- 

 velopment is to a large extent justifiable." Thus the term 

 Palingenesis (or paleogenesis) has been suggested for this in- 

 herent, if partial, tendency of descendent ontogenies to 

 recapitulate ancestral ontogenies. 



Although Cuvier is usually thought of as the father of 



comparative anatomy, or at least its first great authority, it 

 is seldom that we refer back to him. Modern comparative 

 anatomy had its real beginnings with Johannes Miiller, 

 (c. 1840), Carl Gegenbaur (1860-1900), and their students. 

 The documentation of the phylogeny of animals began with 

 Gegenbaur and Haeckel (1864). German biologists were the 

 great contributors at this period and one cannot avoid re- 

 ferring to or using eponyms honoring such names as Meckel, 

 Gaupp, Rathke, Butschli, Furbringer, Wiederscheim, and 

 Gadow. Of like stature among the English were Owen, 

 Balfour, Huxley, and Parker. More recent are Kerr, Good- 

 rich, and de Beer. 



The new comparative morphology did not have easy 

 going. It became embroiled in argumentation over terms. 

 The interpretation of homology was critical since homologs 

 reveal phylogeny, or accepted phylogenies indicate homologs. 

 Gegenbaur related homologs to embryological development. 

 To be homologous, structures must be developed from exactly 

 corresponding rudiments of the embryo. Such a definition 

 resulted in the belief that the pectoral limbs of vertebrates, 

 since they arise at different somite levels, are not homologous 

 but rather parallel developments. This concept was devel- 

 oped and applied to many anatomical details by Furbringer. 



The basis of homology should be the concept of a con- 

 tinuum of living things through time. The pectoral limbs are 

 the saine, although their somite level may be altered — even 

 within a species or on the two sides of an individual. The 

 concept has become entangled with somite differences, germ 

 layer differences, and reorganizations of body plan affecting 

 general position and interrelationships. Further confusion 

 is adduced by considering serially repeated structures, such 

 as vertebrae. From all of this two real factors emerge. First, 

 the degree of homology between two compared structures 

 can range from complete to nil; an arbitrary cut-off point 

 must be decided upon. .Secondly, parallel developments are 

 involved. Recent studies suggest that parallel changes took 

 place in several lines of reptiles, resulting in mammal-like 

 forms. These changes involved conversion of some of the 

 lower jaw bones into middle-ear bones and a concomitant 

 change in the articulation of the lower jaw. This parallelism 

 is revealed against a background of differentiating detail. 

 Technically, parallel structures are not homologous, but one 

 wonders whether "increasing precision of definition," as a 

 scientific goal, is wholly desirable if it increases the number 

 of areas of disagreement. 



At the present time, comparative morphology is enjoying 

 a rather pleasant period of productivity traceable to a num- 

 ber of sources. Morphological studies of fo.ssil fishes by 

 Stensio ( 1927 to the present), Save-Soderbergh, Jarvik, Neil- 

 sen, and Lehman, along with similar studies of many kinds 

 of vertebrates by D. M. S. Watson, Gregory, Westoll, and 

 Romer, have opened up new vistas as to the ancestry and the 

 nature or direction of evolution of the vertebrates. Renewed 

 interest in the quantitative aspects of the speciation process 

 has focused attention on the species and subspecies in terms 

 of time and geographic distribution. Study of the origin of 



INTRODUCTION: AN HISTORICAL SKETCH 



