ANIMAL LIFE AND EVOLUTION 337 



tween them. Among the thousands of worker bees in the 

 same hive, all produced from eggs of the same queen, all reared 

 under the same conditions, and all looking superficially very 

 much alike, the trained eye of the student of variations has no 

 difficulty in discovering differences in size, color, character of 

 wing venation, number of wing hooks, etc., etc., differences 

 indeed in the condition of any part or attribute carefully 

 studied. The variations among the children in one family, 

 among the puppies or kittens in one litter, or among human 

 individuals or dogs or cats in general, are readily apparent, 

 because we are familiar with the human and dog and cat 

 bodies and attributes and readily recognize differences among 

 them. But these differences are no less real and discoverable 

 among the individuals of any other animal species. 



These variations have been determined to be of two kinds. 

 One kind, called acquired, or fluctuating, is produced chiefly 

 by the inevitable environmental differences, kind and amount 

 of food, etc., to which the different individuals of a species or 

 even of one brood of the species, are subjected during their 

 development from embryo to adult. These fluctuating varia- 

 tions are apparently not directly inherited, and hence probably 

 have little to do with determining evolutionary change. They 

 may have some importance in aiding or hindering the individual 

 possessing them in their "struggle for existence," and by help- 

 ing to save or lose the lives of these individuals determine who 

 among them shall live to produce offspring and who shall not. 

 But even those saved by the possession of a favorable fluctu- 

 ating variation will not necessarily hand it on to their offspring, 

 and thus impress it on a future race. 



Variations of the second kind are called congenital, by which 

 is meant that they depend primarily upon the character of 

 the germ cells from which the new individuals are produced, 

 and tend to appear whatever may be the character of the 

 environment during development. These variations are 

 directly heritable, that is, they are handed on to the offspring 

 of the individuals possessing them. What causes them in the 

 first place, what determines that the fertilized egg cells produced 

 by different individuals of the same species shall differ among 



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