xxiT PREFACE. 



cation. If a species be gifted with acute hearing, and the move- 

 ments of the ear-drum require several ossicles^ these, like the 

 labyrinth, grow to full size in the embryo, appropriating the 

 blastema of the contiguous hsemal arch, and projiortionally re- 

 ducing, by aiTesting the development of, the pleurapophysis of 

 such arch. The inherited tendency to a sj^ecial or specific form 

 which thus influences early developments and growth of parts has 

 misled some who ha^'e mistaken such for homological or archetypal 

 characters. But the determination of these characters is arrived 

 at by other routes of research ; and, so reached, such determination 

 serves to explain many of the jihenomena of development which 

 otherwise would remain as mere empirical facts. 



Embryology, e.g., shows that in the human fcetus the sternum 

 is developed from a series of ossific centres (Vol. II. p. 55.5, fig. 

 SGi), whilst the co-articulated cla\dcle — as long a bone — is de- 

 veloped from a single ossific centre, and a contiguous rib, though 

 of greater length, is also hardened from a single ossific centre ; 

 but embryology affords no explanation of the reason of such dif- 

 ference. That is afforded by a knowledge of the archetype 

 skeleton, which teaches that the sternum — reckoned as a sino-le 

 bone in anthropotomy — consists of a series of vertebral elements, 

 but that the rib and the clavicle are single elements. 



Embryology shows that the canon-bone of a ruminant, re- 

 garded as a single bone by the veterinarian, is developed from five 

 ossific centres ; two on the same transverse line near the middle, 

 one on the upper, and a pair which soon coalesce at the lower end. 

 But no clue is afforded to the signification of these several cen- 

 tres : embryology is no criterion of their homologies ; these are 

 determinable on other grounds or ' ways of anatomy.' 



A knowledge of the ' Nature of Limbs,' derived from homolo- 

 gical studies leading to a recognition of the archetype, could alone 

 determine that two only out of those five centres represent dis- 

 tinct bones in the typical pcntadactyle foot of the mammal ; the 

 rest having no such signification, but serving to jierfcct the ulti- 

 mate growth as ' epiphyses.' So likewise with the collar-bone 



