INTRODUCTION 



The intention of this book is to present a balanced account 

 of the theoretical matter of animal biology. Botanical results 

 are only noted in so far as they bear upon general biological 

 science. All detail that is not illustrative of what may be called 

 the principles of biology has been omitted, however attractive it 

 may be. Doubtless the method of treatment leads to sins of 

 omission, but it is hoped that these are not really important 

 ones. There are vast masses of data in all departments of 

 biology that have but slight relevance in general discussions, 

 and the non-professional reader loses little by a careful selection 

 of the essentials of the science. It must, however, be admitted 

 that some parts of zoology present great difficulties to the reader 

 who has not undergone the discipline of practical laboratory 

 work on " animal types." I have tried to minimize these diffi- 

 culties by giving a summary account of the forms of life looked 

 at from the morphological standpoint, and this can easily be 

 supplemented by study of the many excellent small books on 

 special aspects of our science. 



The present seems to be a very opportune time for making 

 a survey of the main results of animal biology. The last forty 

 years have been the period of a revolution in physics such as 

 has no parallel, at any time, in biology. During this period the 

 main lines of investigation in the biological sciences have been 

 these : (i) biometry ; (2) the investigation of the cell and develop- 

 ing embryo, and (3) genetics and biochemistry. Biometrical 

 methods, as they were elaborated by Weldon, Galton and Karl 

 Pearson, have been so far perfected that they are now well 

 ahead of the observational side of statistical biology, a line of 

 research which is certain to be very greatly amplified in the 

 future. We have, perhaps, now nearly come to a halt with 

 respect to the pure morphology of the cell : further advances 

 along that direction may well depend upon the improvement of 

 methods of micro-dissection together with the development of 



