CHAPTEE LIX 



REPRODUCTION, DEVELOPMENT, GROWTH AND DEATH 



THE history of the fully formed animal from the ovum, and a 

 description of the origin and formation of its tissues and organs, 

 constitutes the portion of biological science known as embryology. 



The scientific discussion of embryology must not be limited to 

 man, however important he may be, but must embrace a wide survey 

 of the whole animal kingdom, because the changes which occur in 

 the embryological history of the highest animals forms a compressed, 

 though in many cases a modified picture of the changes which have 

 taken place in their historical development from lower types, during 

 the long and misty ages of the past. 



To attempt an adequate discussion of this large subject would 

 mean the writing of a book as long as the present volume. It would 

 lead us far into biological fields, and also into discussions of a 

 philosophical and hypothetical nature which would be quite out of 

 place in a physiological text book. All we can do is to put down 

 the important facts, and specially to dwell on those which have a 

 physiological bearing. Evolution has in the past been specially 

 studied from the anatomical point of view; but it has its physio- 

 logical counterpart ; for as structures increase in complexity, so also 

 does function become correspondingly differentiated and varied. 



The great problem of evolution is associated with the immortal 

 name of Darwin. He showed that although the offspring resembles 

 the parents there are always certain slight variations which may be 

 transmitted to future generations, and in their turn these are 

 magnified or admixed with other variations in each succeeding 

 generation. In the struggle for existence those will survive in 

 which the variations are beneficial and helpful, so that in the end the 

 fittest will live on. The piling up of variations which in themselves 

 may be trivial thus leads to the development of new types and 

 species, and this, prolonged over immense periods of time, has led to 

 the evolution of complex from simple forms of life. 



The explanation of heredity is a subject on which much difference 



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