3 8 THE GROWTH OF THE BRAIN. 



size of which is least limited by age the possibility of 

 development is more persistent. 



Connected with these facts are those bearing on the 

 variations of size in animals of the same family or 

 species. It is worthy of note that there are many cases 

 where a type of vertebrate animal appearing at several 

 geological horizons has the last representatives more 

 bulky than the first, while the extremes arc connected 

 through the different horizons by a series of forms inter- 

 mediate in size. Extinction in these cases apparently 

 overtakes the type when its representatives have 

 reached the greatest size, and the suggestion here is 

 that species, like individuals, have a period of growth 

 followed by specialisation, which latter is in the end 

 the cause of their extinction, through the loss of adapta- 

 bility to new conditions. On the other hand, some 

 invertebrates slowly deteriorate in the course of their 

 geological history perhaps through an unequal struggle 

 with changed conditions, which, though damaging, were 

 not sufficient to exterminate them. 



The influences affecting the animal from without are 

 expressed by the term environment ; while from within 

 there is the complex of influences called heredity. 

 Probably dependent on external conditions are the 

 peculiarities of faunas on the smaller islands, one feature 

 of which is the small size of the animals there found. 

 So, too, the experiments of Yung 1 in rearing tadpoles 

 indicate the relation between the capacity of the en- 

 closing vessel and the possible size of the tadpole which 

 was being reared in it. Semper and De Varigny 

 obtained similar results with the pond snail (Lymnsea) 2 

 by growing them in different quantities of water, varying 



1 Yung, Compt. Rend., 1885. 



2 Semper, Animal Life, 1881. De Varigny, Journ. de Vanat. et 

 physiol., Paris, 1894. 



