i 5 8 



THE QUARTERLY REVIEW OF BIOLOGY 



A), just as of old. As an extreme illustra- 

 tion one might select Reichert's demon- 

 stration by embryological means of the 

 homologies of the auditory ossicles of 

 mammals, which proved to be a modifica- 

 tion of the whole hinder portion of the 

 lower jaw, as it exists in reptiles, as well 

 as of the separate quadrate. The striking 

 nature of this case leads Gaupp to 

 remark: "Thus we have here in the his- 

 tory of the auditory ossicles of mammals 

 a highly remarkable case of change of 

 function, perhaps the most remarkable 

 case in the whole realm of vertebrate 

 morphology; skeletal parts which pre- 

 viously formed important members of the 

 mandibular apparatus, are in the mammal 

 alienated therefrom and assigned to an 

 entirely new duty." In this particular 

 instance, where a series of articulating 

 bones commences and ends as a bone- 

 linkage, it is almost unnecessary to point 

 out that as a form of intrinsic physio- 

 logical mechanism it remains almost the 

 same. To this day the question has 

 scarcely been raised whether identity in 

 respect of Function B should not be 

 incorporated as an essential element in our 

 predication of likeness by homology. 



HOMOGENY AND HOMOPLASY 



As we have just seen, morphologists 

 were not always careful to distinguish 

 the homology based on comparison of 

 adult characters from that which is 

 embryologically determined. By inject- 

 ing into positional homology all the 

 implications of the theory of descent, they 

 persuaded themselves that when they 

 determined homologies after the old 

 fashion, they were establishing embry- 

 ological homology. In an important 

 communication published in 1870 Ray 

 Lankester drew attention to the danger 

 of this procedure. In order to rectify 

 matters he proposed at the same time a 



reform of the terminology, his statement 

 being somewhat as follows: 



He first suggested that organs may 

 appropriately be called homogenetic if the 

 common possessors are derived from 

 ancestors that possessed the same organ. 

 But homology had not infrequently been 

 based upon considerations of pure posi- 

 tion. Thus, the four cavities of the 

 bird's heart had been said to be homolo- 

 gous with the four cavities of the mamma- 

 lian heart, in spite of the fact that the 

 common ancestors of mammals and birds 

 had in all probability but three heart 

 cavities, and in spite of the further fact 

 that the right ventricle of a bird's heart 

 does not develop in the same way as the 

 right ventricle of a mammalian heart. 

 Again, certain muscles in the limbs of 

 Sauropsida were said to be homologous 

 with other muscles in the limbs of 

 Mammalia, although the presumption was 

 that no such muscles were present in the 

 limbs of the common amphibian ancestors. 

 (Here Lankester was following Huxley's 

 now discredited derivation of mammals 

 from an amphibian and not from a rep- 

 tilian stock.) "Again," says he, "it may 

 perhaps be admitted that the common 

 ancestors of the Osseous Fishes and 

 Mammalia had a skull of decidedly 

 undifferentiated character, with a much 

 less amount of differentiation than is 

 observed in the skulls of either of these 

 groups. It is only in so far as they have 

 parts represented in the common ancestor 

 that we can trace homogeny in these groups; 

 and yet the homology of a vast number of 

 bones in the skull of the two is discussed 

 and pointed out." He further instanced 

 the serial homologies, in which a corre- 

 spondence is traced in detail between the 

 structures composing, say, the fore-limb 

 and those composing the hind-limb of one 

 of the higher vertebrates. His conclusion 

 was that something over and above 



