60 HEREDITARY CHARACTERS 



that have been put forward as explanations of the occur- 

 rence of variation, it is necessary to consider in some detail 

 the nature of the phenomenon itself. 



In the great majority of cases the differences between 

 parent and offspring, and between the offspring of the same 

 parents, are small, often being limited in the case of the 

 higher animals, as far as superficial examination goes, to 

 colour or to differences in size generally or in some par- 

 ticular part. The differences most familiar to us are of 

 course those occurring among other human beings, where, 

 for instance, besides frequently observing a resemblance 

 between father and son, we are also able to see considerable 

 differences. This is due to the fact that we are trained to 

 observe differences between individuals of our own kind with 

 an accuracy that is impossible in the case of other animals 

 or of plants. The evidence of trained observers shows us 

 that variations occur even among microscopic organisms. 

 Besides occurring in microscopic organisms, variations also 

 occur in new individuals that have been produced partheno- 

 genetically. Dr. E. Warren observed several generations of 

 one of the Entomostraca Daphnia magna a minute 

 crustacean which produces many generations without any 

 fertilisation taking place. 1 He found " considerable varia- 

 bility" even among the children of the same brood. 

 Weismann also found variations among the partheno- 

 genetic offspring of Cypris reptans. Warren's recorded 

 differences were obtained by measurements ; Weismann 's 

 differences were in colour. 2 



Another very good example of variation occurring in 

 parthenogenetic organisms is afforded by the Polyzoa. 



" The Polyzoa are a class of aquatic organisms of world- 

 wide distribution and including a large number of species. 

 They occur both in fresh water and in the sea, and the 



1 Warren, E., "An Observation on Inheritance in Parthenogenesis," Proc. 

 Hoy. Soc., vol. Ixv., 1899. 



2 Weismann, A., The Germ Plasm: a Theory of Heredity, translation, 

 1893 German edition, 1892. 



