VI PREFACE TO THE REVISED EDITION. 



philosophical as it might be in theory, it had defects in 

 practice. 



All the simplest forms of life, which are easily accessible, 

 are of very minute size and their study involves the use 

 of high microscopic powers. The student who begins with 

 them is therefore not merely introduced suddenly into a 

 region in which everything is new and strange, but he has 

 to familiarize himself with the use of unwonted means of 

 exploration. By taking this road, the teacher (to whom the 

 world of the microscope is so familiar that he is apt to 

 forget its strangeness to students) sets himself against one 

 of the soundest canons of instruction, which is to proceed 

 from the known to the unknown, and from familiar methods 

 of learning to those which are strange. 



After two or three years' trial of the road from the simple 

 to the complex, I became so thoroughly convinced that the 

 way from the known to the unknown was easier for students, 

 that I reversed my course, and began with such animals as 

 a Rabbit or a Frog, about which everybody knows something, 

 while their anatomy and physiology is illustrated by in- 

 numerable analogies with those of our own bodies. From 

 this starting point we proceeded further and further into 

 the unfamiliar regions of invertebrate organisation until we 

 reached the border region between animals and plants, 

 whence there was a natural and easy ascent to the most 

 complicated vegetable organisms. 



This order is followed in the present edition; which is 

 greatly improved by the addition of the Earthworm and the 

 Snail in the series of animal, and of Spirogyra in the series 

 of vegetable, types. 



