THE SHAPES AND SIZES OF ANIMALS 141 



become intensified, attaining their maximum in birds 

 and mammals, the two highest groups of animals. 



I can hardly conclude this part of my address 

 more fittingly than by the following quotation from 

 Herbert Spencer who, in his " Principles of Biology," 

 has discussed in most philosophical fashion, and 

 with far greater thoroughness than I can pretend to 

 here, the laws regulating the shapes of animals. "The 

 one ultimate principle," says Spencer, " that in any 

 organism equal amounts of growth take place in 

 those directions in which the incident forces are 

 equal, serves as a key to the phenomena of mor- 

 phological differentiation. By it we are furnished 

 with interpretations of those likenesses and un- 

 likenesses of parts which are exhibited in the 

 several kinds of symmetry ; and when we take into 

 account inherited effects wrought under ancestral 

 conditions, contrasted in various ways with present 

 conditions, we are enabled to comprehend, in a 

 general way, the actions by which animals have 

 been moulded into the shapes they possess." 



Passing from the consideration of the shapes to 

 that of the sizes of animals, is very like turning 

 from a well-made road into a ploughed field, across 

 which progression becomes not only slow but 

 difficult and irregular. Hitherto the problems 

 concerned with the sizes or magnitudes of animals 

 have received but very scant attention, and we are 

 not only ignorant of the principles and laws that 

 govern them but of the directions in which to 

 seek for these principles. Indeed we have at 

 present but a very limited number or range of 



