no 



HOW ANIMALS DEVELOP 



gets slower and slower as the animal gets older. 

 Fig. 27 shows how much the proportions alter as a 

 baby grows up to be a man. There is often an 

 evolutionary significance in these changes of pro- 

 portion and we can show, in the evolutionary tree of 



Fig. 27. — The changes of proportion during growth, (a) and (c) are 

 human and gorilla foetuses, (b) and {d) an adult man and gorilla. 

 All the figures are adjusted to have the same sitting height. Notice 

 that in man the upper part of the body retains more nearly the foetal 

 proportions, whereas in the gorilla the legs remain more unchanged 

 while the head, and particularly the brain-case, becomes very small. 

 (After Schultz.) 



the horses, for instance, which we know from fossils, 

 how one species has changed into another by an 

 alteration of the rates of growth in different direc- 

 tions. Professor Huxley has recently written a 

 fascinating book on this subject called Problems of 

 Relative Growth. 



The rates of growth of the different parts of a 

 young animal or embryo at any given stage are not 

 quite rigidly fixed, but can be altered for two 



