x PREFACE. 



of the mineral or crystalline species, but is subject, like vital 

 growths, to occasional malformation. 



As this principle of growth by multiplication of like parts is 

 manifested more commonly and extensively in plants, it is illus- 

 trated in the ' Invertebrate Volume' 1 under the term ' Vegetative 

 —-^ Repetition.' ( In the vertebrate series it is exemplified by the 

 hundreds of similar teeth in the jaws of many of that low class 

 (Pisces) in which true dentinal teeth first appear in the animal 

 kingdom. The numerous and similar many-jointed terminal divi- 

 sions of the pectoral limbs of the fishes thence called ' Rays,' the 

 multiplied similar endoskeletal segments of the vertebral column of 

 these and other cartilaginous fishes, of muraenoids and serpents, are 

 likewise lingering exemplifications of the low irrelative principle 

 of development. 



In the vertebral embryo the first appearance of the parts of the 

 skeleton, in gristle or bone, is a segmental one ; in fishes the mus- 

 cular system shows much, and in all Vertebrates a little, of the 

 like seo-mental constitution of the trunk : the same idea is suo-o-ested 

 by the symmetrical and parial origins of the nerves, and phy- 

 siologists have mentally recognised a corresponding segmental 

 condition of the myelon or spinal chord, which is visibly exem- 

 plified in certain fishes. But these appearances are concealed by 

 the general tegument ; not exposed, as in the Articulates, in which 

 the segmented skeleton is at the same time tegument. A Verte- 

 brate may be defined as a clothed sum of segments. But in this 

 highest province of the animal kingdom growth by repetition of 

 parts rapidly gives place to the higher mode of development by 

 their differentiation and correlation for definite acts and complex 

 functions. Nevertheless, I am constrained by evidence to affirm 

 that in the vertebrate as in the invertebrate series there is mani- 

 fested a principle of development through polar relations, work- 

 ing by repetition of act and by multiplication of like parts, con- 

 trolled by an opposite tendency to diversify the construction and 

 enrich it with all possible forms, proportions, and modifications of 



1 Op. cit. 2nd od. p. 541. 



