416 GENERAL ZOOLOGY. 



or (3) between the individual and its environment. In 

 what way is difference in fitness brought about? 



Variation. Animals and plants are continually vary- 

 ing. Children of the same parents differ from their 

 father and mother and from each other in size, features, 

 color of eyes and hair, as well as in mental characteristics. 

 The differences may be slight, but still they are notice- 

 able. So, too, the chickens of a brood, the kittens of a 

 litter, the caterpillars from an egg cluster are not exact 

 repetitions of one another, but each has its own indi- 

 viduality. The same is true of every animal and every 

 plant. 



These variations between the individuals of a species 

 would naturally be in different directions. Some individ- 

 uals would differ from the rest in such a way as to fit 

 them better for their surroundings. They might have; 

 keener senses or quicker motions and thus be better 

 adapted to obtain their food or to escape their enemies,. 

 On the other hand others might vary in such a way as 

 to be handicapped in the struggle for existence. In such 

 a severe contest as is going on, where only a small fraction 

 of a generation can survive, any advantage, however slight, 

 may decide the question of life or death. It logically 

 follows that in the long run only those whose variations 

 have been in a beneficial direction will arrive at maturity, 

 and that only those which have thus varied to some ex- 

 tent, however slight, from their ancestors, will produce the 

 next generation. 



There may be two kinds of variation. In the first the 

 modifications from the ancestor may arise from the 

 germ-cells (eggs and spermatozoa) and be in no way de- 

 pendent upon external conditions. Thus the markings 

 of a litter of kittens, the differences between twins, are 



