EXISTING ORGANISMS— PHYSIOLOGY 103 



the circulation to the developing embryo, instead of secreting it 

 as an addition to the egg. Here development is carried to a state 

 of partial perfection, and the young animal is then carried in a 

 pouch on its mother's abdomen and nourished with milk until 

 able to shift for itself. Finally, the Eutheria, or true mammals, 

 illustrate an elaboration of the same process. The connection of 

 embryo with adult is so intimate that it is looked upon as essen- 

 tially a parasitic relationship, and development proceeds not only 

 to morphological completeness, but through a considerable degree 

 of growth before parent and offspring are separated l:)y ])irth. 



Throughout the multitude of functions of the living organism 

 of which these are only a f(nv illustrations, occur the fundamental 

 relationships which have here been emphasized. From simplest to 

 most complex they are no more than the five fundamental func- 

 tions, yet in these and in the complexities of their distribution 

 among the parts of the specialized organism we see one more evi- 

 dence of common relationship of all living things, and of lesser 

 relationships of various kinds among the different groups. 



Structure, development, function, and the classification which 

 we have derived from the study of these and other things, all 

 point to the truth which is so well established in modern science, 

 that all living things are related. This much cannot be logically 

 doubted; it is the function of evolution to explain their relation- 

 ship. 



Summary. The relationship of organisms is as well shown by 

 their physiological processes as by their structure and develop- 

 ment. Plants in general have one fundamental process of metab- 

 olism, animals another. Some organisms carry on both types 

 and others, the colorless plants, form a connecting link between 

 the green plants and animals. Exceptions to these normal proc- 

 esses make it all the more evident that similarity means funda- 

 mental relationship. The activity of the endocrine glands is a 

 striking evidence of relationship among animals. Blood tests, 

 based on the immunizing reaction of the blood of an animal to 

 that of another species, have been worked out so extensively that 

 they not only illustrate the fact but also the varying degrees of 

 relationship. The process of cell reproduction, mitosis, links all 

 organisms, and in the reproduction of individuals we find indica- 

 tions of less extensive relationship within phyla. The vertebrates, 

 for example, show a gradual transition of reproductive functions. 



