354 Appendix B 



to individual plasticity,' writes the latter, ' yet these modifications 

 afford the conditions under which variations ^ of like nature are 

 afforded an opportunity of occurring and of making themselves 

 felt in race progress.' 



" The significance of this principle is clearly seen w^hen it is 

 studied in connection with the family system that prevails 

 among the higher classes of animals, which feed and tend their 

 young and to some extent educate them. Among social species 

 it rises to still greater importance. In the light of this new 

 principle the tending of the young by the parents is not merely 

 a system by which waste is prevented ; it is also a system which 

 prevents a species from deviating widely from the line of 

 development that it has begun to follow. 



" I shall now try to make clear, mainly by examples, how the 

 principle works. And first I shall try to show its operation 

 when parental affection is not present to bring out its further 

 possibilities. It may be stated thus : A congenital variation, 

 in itself too minute to afi^ect the question of survival^ may gain 

 selectio7i-vaIue tJwough exercise. The variation havijig thus been 

 saved by exercise, further variations in the same directiofi may occur. 



" The ancestors of the amphibians lived throughout their lives 

 in water, breathing the oxygen dissolved in it by means of gills. 

 Now individuals in whom a rudimentary lung appeared, a pouch 

 opening from the oesophagus, might develop the breathing 

 capacity of this rudiment by coming frequently to the surface 

 and inhaling air, or by getting out on to the bank either to rest 

 or to escape from enemies. Then there might arise a terrible 

 emergency such as comes to many 'water breathers,' if they 

 live in fresh-water pools ; there might be a drought causing the 

 pools to dry up. At this crisis some individuals are saved 

 by their lungs. They have so far developed their makeshift 

 pouches by exercise that they are able, though not wdthout 

 strain and discomfort, to become exclusively air-breathers, till 

 at length rain comes or they have made their way to another 

 pool from which the water has not evaporated. If there is a 

 succession of such droughts, there will be a further selection 

 1 Italics mine (Hcadlcy). 



