26 THE ANATOMY OF INVERTEBRATED ANIMALS. 



parts, which modification is carried to a still greater extent 

 in C. 



In other words, A, B, and C, differ from one another in 

 the same fashion as the earlier and later stages of the em- 

 bryo of the same animals differ ; and, in successive epochs, 

 we have the group presenting that progressive specialization 

 which characterizes the development of the individual. Clear 

 evidence that this progressive specialization of structure has 

 actually occurred has as yet been obtained in only a few cases 

 (e. g., Equidai, Crocodflia), and these are confined to the 

 highest and most complicated forms of life ; while it is de- 

 monstrable that, even as reckoned by geological time, the pro- 

 cess must have been exceedingly slow. 



Among the lower and less complicated forms, the evidence 

 of progressive modification, furnished by comparison of the 

 oldest with the latest forms, is slight, or absent ; and some 

 of these have certainly persisted, with very little change, 

 from extremely ancient times to the present day. It is as 

 important to recognize the fact that certain forms of life have 

 thus persisted, as it is to admit that others have undergone 

 progressive modification. 



It has been said that the successive terms in the series of 

 living forms are analogous in all parts of the globe. But the 

 species which constitute the corresponding or homotaxic terms 

 in the series, in different localities, are not identical. And, 

 though the imperfection of our knowledge at present pre- 

 cludes positive assertion, there is every reason to believe that 

 geographical provinces have existed throughout the period 

 during which organic remains furnish us with evidence of the 

 existence of life. The wide distribution of certain Palaeozoic 

 forms does not militate against this view ; for the recent in- 

 vestigations into the nature of the deep-sea fauna have shown 

 that numerous Crustacea, Echinodermata, and other inver- 

 tebrate animals, have as wide a distribution now as their ana- 

 logues possessed in the Silurian epoch. 



III. Physiology. 



Thus far, living beings have been regarded merely as 

 definite forms of matter, and biology has presented no con- 

 siderations of a different order from those which meet the 

 student of mineralogy. But living things are not only natural 

 bodies, having a definite form and mode of structure, growth, 

 and development. They are machines in action ; and, under 



