PALEOZOIC TIME — CARBONIC. 717 



(6) From large aquatic structures to smaller and more concentrated terres- 

 trial structures. 



2. Approximate parallelism, in many cases under any tribe, between the 

 geological succession of structures and embryological succession in the develop- 

 ment of living organisms. — On this subject see the remarks on page 401. 



3. Degeneration. — (1) In cases where progress is upward, or where there 

 is no manifest decline in grade : (a) Degeneration of an organ to a more 

 primitive form ; (6) diminished size and often complete disappearance of an 

 organ (either from disuse, or in consequence of accelerated enlargement in 

 associated organs). (2) In cases of decline in grade: Degeneration widely 

 in a structure through changes that have the reverse order of those enum- 

 erated in the preceding paragraphs, leading often through youth-like to 

 embryonic forms ; producing low-grade structures that are nearly normal in 

 form and activity ; also lower down, variously defective structures, sluggish 

 in movement; and at the extreme limit of degradation in Invertebrates, 

 structures that are incapable of locomotion after leaving the young stage ; 

 also, where an animal becomes aquatic among Vertebrates, producing struc- 

 tures which retain activity, become urosthenic and multiplicate, and often 

 have great length of body and large size. 



Degeneration has its limits. Degenerate Mammals are mammalian in 

 their more fundamental characteristics. Degenerate Crustaceans are Crus- 

 taceans still, as they show in their embryonic development. 



4. From diffuse structures to concentrated. — Since the brain or cephalic 

 ganglion, besides being the source of physical energy, and the chief seat of 

 sensorial energy, is the center of control of all the forces of the structures 

 except the involuntary, concentration consists in a shortening of the radius 

 of control, or the distance through which it has to act. Compare a Lobster 

 with the highest of Crustaceans, a Crab ; or a Crab with its superior, an Ant. 

 Some of the cases included are the following: (1) From a much elongate 

 structure — the elongation chiefly posterior — to an abbreviated structure. 

 (2) From a multiplicate structure, or one having an excessive or indefinite 

 number of body segments, pairs of limbs, articulations of limbs, etc. — a pre- 

 vailing feature of Articulates of the early Paleozoic — to one consisting of a 

 normally limited number of such parts and usually also an arrangement of 

 these parts in two or three groups. (3) From a structure having the pos- 

 terior part of the body the chief locomotive organ to one having regular pairs 

 of limbs as the organs of locomotion, and having these pairs of limbs situated 

 anteriorly in the structure ; in which case the structure is styled podosthenic 

 (from the Greek for foot and strong) . (4) From a structure in which the 

 posterior pair of limbs in Vertebrates is the strongest, and which is there- 

 fore merosthenic (so-named from the Greek for thigh and strong), to one in 

 which the anterior feet are the strongest, — a structure styled prosthenic. 



