468 SUMMARY AND CONCLUSION. 



is dependent on antecedent structure, and that is automatically created in the 

 process of growth and organic readjustment. 



There are therefore various aspects of evolution; some are over-emphasized 

 by one school of biologists, and ignored by others, because the creative factors 

 have widely different values at different periods of evolution, and in different 

 fields of investigation. 



There is no master key to evolution. It does not always move in the same 

 way; not always by continuous, nor always by discontinuous variations; it is not 

 always direct, orthogenic, determinate; not always indeterminate; heredity is not 

 always a controlling factor, nor does it always control in the same way; neither 

 does the external environment, nor use, nor disuse, nor natural selection. All 

 are real factors and all have doubtless played some part in the grand total of 

 results, but each has a different value, more here, less there, and these values 

 have changed with the progress of evolution. They are as varied as life itself. 



FIG. 307, 308. Diagrams illustrating the principal stages in the evolution of segmented animals (syncephalata) . 

 They illustrate: a. The spasmodic increase in the number of metameres, the advent of each new group (tagma) 

 marking a distinctly higher level in evolution, of class, sub-class, or divisional value, b. The approximate historic 

 period at which new functions and organs, demanded by the new internal conditions, make their appearance; e.g., 

 circulatory, respiratory, locomotor. c. The initial location of the most important functions and organs, d. The most 

 important changes in the location of functional centers, due to the transfer of organs to other regions, or to their 

 degeneration or atrophy, and the appearance of new organs elsewhere to take their place. The substitution of 

 new organs and functional centers for the old is apparently always in a haemad, or caudad, direction, never cepha- 

 lad, or neurad. The most striking change in the location of old organs is the transfer of the appendages and as- 

 sociated parts (visceral arches, nerves, muscles, and ganglia in an anterior haemal direction, the process beginning 

 with the oral or anterior thoracic arches of primitive crustacea and attaining completion in the mammals, with 

 the transfer of all the branchial arches to the anterior haemal surface of the head. One of the principal causes 

 of this change in the position of organs is the atrophy of all the organs on the corresponding haemal surface of 

 the head. The most striking illustrations of local atrophy, and the formation in the younger, more posterior 



metameres, of new organs or parts of organs serving the same purpose are shown by the locomotor, L, sexual, 



excretory, X, digestive, D, and circulatory organs, C. e. The substitution of one organ for another. The most 

 striking illustration is the closing of the old mouth and stomodaeum, and the formation of a new opening into the 

 mesenteron in the region of the "dorsal organ," i.e., on the haemal surface of the procephalic and anterior dia- 

 cephalic region. Other examples are the substitution of lungs for gills; and of local expansions of the lateral or 

 pleural folds of the trunk, that serve as balancing, supporting, and locomotor organs, in place of the cephalic 

 appendages. / The permanency and very great antiquity of the more anterior cephalic organs is strikingly shown 

 by the procephalic structures, such as the median and lateral eyes, olfactory organs and their ganglia, the primi- 

 tive "hemispheres," cerebellum and stomodaeum. g. The most striking innovation is the perforation of the walls 

 separating gill sacs and enteric diverticula. h. The rise and decline of metamerism. Metamerism is never complete 

 or perfect at any phylogenetic or embryonic period, or in any region. It attains its highest expression in the mid- 

 body region of the higher arachnids and is but very incompletely expressed in the anterior cephalic and 

 caudal regions. The decline of metamerism begins in the higher arachnids, ostracoderms, and primitive verte- 

 brates, and makes its appearance first, and in the most marked degree, in the oldest or most cephalic regions, and 

 more on the haemal than the neural side. That is, it follows the primary axes of growth and structural differ- 

 entiation, i. Result. Each new local growth, atrophy, transfer, substitution, or innovation, of parts is interlocked 

 with all the others, inevitably creating new conditions pregnant with new structures and activities. The process is 

 most conspicuously manifest in the gradual creation of the mammalian "head" and "body," with the old struc- 

 tural units in a totally new and different organic relation from the initial one. The final, permanent relation of 

 part to part, and of the new to the old, is a logical, inherently necessary one, the most important internal factors 

 in bringing it about being priority of origin, the imperative demands for intercommunication and distribution of 

 products, mechanical balance, coherency, and stability. 



The following capital letters signify the location of the principal functions: C, Cardiac, or circulatory; 



D, digestive; G, gustatory; L, locomotor; M, masticatory; R, respiratory; X, excretory; , sexual. Other letters 



as before: A, nauplius stage; B, ostracode; C, cladoceran; D, merostome; E, transitional; F, larval fish; G, am- 

 phibian; H, mammalian. 



