PREFACE 



IN this book I have endeavored to present in an 

 elementary way some of the fundamental generali- 

 zations that are the product of modern research in 

 biology. The artificial division between the study 

 of plants and that of animals is one that is becom- 

 ing increasingly difficult to maintain, inasmuch as 

 some biological principles are best illustrated by 

 phenomena in the plant world, others by those of 

 the animal world. I have tried, therefore, to utilize 

 both aspects of the subject and to draw my illustra- 

 tive material impartially from both kingdoms. 



The practice that insists upon the student getting 

 his knowledge of natural science at first hand needs 

 nowadays no justification. The laboratory method 

 of study has shown itself to be not only the best 

 means of acquiring a concrete and accurate knowl- 

 edge of the science studied but also a primary pre- 

 requisite for those habits of thought that are essential 

 to what has come to be known as the " scientific 

 method." Nevertheless in Biology the field is so 

 broad and so varied that the student is very likely 

 to lose sight of the fundamental principles that 

 underlie all living nature. Moreover, these princi- 

 ples do not grow out of the laboratory work so 

 obviously nor are they so easily demonstrated by 



vii 



