viii PREFACE 



conception. We observe many of the means by which energy 

 is stored, and some of the compHcated methods by which it 

 is captured, protected, and released. We shall see that highly 

 evolved organisms, such as the large reptiles and mammals 

 and man, present to the eye of the anatomist and physiologist 

 an inconceivable complexity of energy and form; but this we 

 may in part resolve by reading the pages of this volume back- 

 ward, Chinese fashion, from the mammaP to the monad, in 

 which we reach a stage of relative simplicity. Thus the or- 

 ganism as an arena for energy and matter, as a complex of in- 

 tricate actions, becomes in a measure conceivable. The 

 heredity-germ, on the contrary, remains inconceivable in each 

 of its three powers, namely, in the Organism which it produces, 

 in the succession of germs to which it gives rise, and in its own 

 evolution in course of time. 



Having now stated the main object of these lectures, I 

 invite the reader to study the following pages with care, be- 

 cause they review some of the past history and introduce some 

 of the new spirit and purpose of the search for causes in the 

 domain of energy. I begin with matters which are well known 

 to all biologists and proceed to matters which are somewhat 

 more difhcult to understand and more novel in purpose. 



In this review we need not devote any time or space to 

 fresh arguments for the truth of evolution. The demonstra- 

 tion of evolution as a universal law of living nature is the 

 great intellectual achievement of the nineteenth century. 

 Evolution has outgrown the rank of a theory, for it has w^on 

 a place in natural law beside Newton's law of gravitation, 

 and in one sense holds a still higher rank, because evolution is 

 the universal master, while gravitation is one among its many 



' Man is not treated at all in this volume, the subject being reserved for the final 

 lectures in the Hale Series. 



