470 SUMMARY AND CONCLUSION. 



stemming the tide of evolution with a scalpel, a rule of conduct, or a dietetic 

 formula. 



It is not surprising that the large expectations of the outdoor naturalist, the 

 cytologist, the experimental evolutionist, and the animal breeder have not been 

 realized. 



The cytologist is too intent on the raw materials of life; his field of operation 

 is both too remote and too narrow to give either measurable detail or perspective. 

 To discover the immediate causes of any given stage in the evolution of the nervous 

 system, or of the endocranium, by a study of chromosomes, or of protoplasm, 

 or by juggling with imaginary hereditary units is as hopeless a task as it would 

 be for the geologist to explain the delta of the Ganges by an appeal to the com- 

 position of cosmic matter. 



The naturalist is bewildered by the amazing detail of the finished product, 

 and so much absorbed in the social organization of the present moment, or in the 

 relation of one plant, or animal to the other, and to the environment at large, that 

 he fails to acquire an adequate historic perspective. 



The experimental evolutionist, for a few hours, or months, arbitrarily nar- 

 rows the environment of an organism, and measures the results, if any, with 

 instruments of precision, or with the aid of the higher mathematics; but he gener- 

 ally ignores, or looks with contempt, on the vast experiments already performed 

 for him, where the laboratory is nature, and the results are expressed in species, 

 genera, and classes. 



The comparative morphologist aims, not merely to trace the identity of 

 changing structures under the disguise of new forms, but to measure the rate of ' 

 these changes, and to seek out the underlying causes that have brought them 

 about. He is heavily handicapped by the lack of materials that can be precisely 

 measured or controlled. But on the other hand there is a certain advantage 



Fig. 309. — Diagram illustrating the phylogeny of the principal subdivisions of the animal kingdom. The 

 numbers indicate approximately the periods at which some of the more important events in the evolution of 

 structures and functions have taken place. 



51, Fixation of balanced internal temperature; 50, decline of yolk volume, and perfection of uterine gestation; 

 49, maximum size of yolk sphere; 48, decline of exoskeleton and notochord and perfection of endoskeleton ; 47; 

 evolution of larynx, cochlea, ear-bones; 46, decline of gills, closure of gill clefts; 45, pectoral and pelvic appendages, 

 supporting and digitate; 44, lungs; heart, three-chambered; 43, second migration to land; 42, elongation, and joint- 

 ing of pectoral fins; 41, air bladder; 40, median fusion of paired jaws; fixation of maxillae; 39, decline of meta- 

 merism; 38, decline of cephalic appendages and rise of lateral folds and paired fins; 37, merging of endo- and exo- 

 skeletal elements, and calcification of endo-skeleton; 36, vertebral rings; heart two-chambered; 360, decline of 

 predatory life; 35, gill sacs unite with, and perforate the gut; 34, leg-jaws; three pairs transferred to the haemal 

 side; 33, neuron continuous; tubular; infolding of the lateral eyes; 32, closure of neurostoma; rise of haemos- 

 toma; 31, increased size of forebrain and cephalic sense organs; 30, increase of endo-skeleton, endocranium, noto- 

 chord and gill cartilages; 29, new generation of caudal metameres; new trunk and tail flexible laterally; 28, 

 postanal concrescence of the germ wall; 27, increase in size of eggs and in the volume of yolk; 26, exoskeleton 

 thickened, trabeculate, cellular, fragmented; 25, notable increase in size and improvement in locomotor and 

 predatory organs; carnivorous; 24, first invasion of land; 23, respiratory appendages, forming infolded sacs; 

 22, parietal eye enclosed in forebrain vesicle; 21, endocranium; 20, leg-jaws, multiple, paired, neural; 19, tagmatism, 

 and linear distribution of function; 18, metamerism perfected; number small; 17, heart; internal circulation; 

 16, rise of excretory organs; 15, body plasma; separation of internal and external media; 14, exoskeleton, water 

 proof, chitenoid, non-cellular; 13, ccelom, telocoel; 12, metamerism perfected; 3 to 7 in number; 11, pursuit and cap- 

 ture; 10, appendages become locomotor, grasping, respiratory; 9, cephalic sense organs, median and lateral eyes 

 and olfactory organs; 8, bilateral symmetry; 7, gastrulation ; 6, radiate symmetry; 5, neuron circumoral; 4, enteron 

 opening directly to exterior; 3, early evolution of tissues; 2, evolution of cells; i, evolution of protoplasm. 



