CHAPTER XXV 

 HEREDITY AND EVOLUTION 



IT will be a matter of common knowledge that the offspring of 

 either plants or animals resemble their parents very closely. 

 This fact, though so obvious, really involves the fundamental 

 principle of inheritance, i.e. the progeny inherit the characteristics 

 of their parents. Thus, if we sow the seeds obtained from a 

 self-pollinated flower of the Foxglove, the numerous resulting 

 seedlings obviously inherit the same general characters. Closer 

 observation, however, shows that there are many minute points 

 of difference which may in the main be related to the fact that 

 the conditions in the seed-bed are not uniform (cf. also Fig. 218). 

 The features presented by any particular individual may, as a 

 matter of fact, be regarded as the outcome or resultant of two 

 sets of factors, being either inherited or due to the effect of the 

 environment. Under environment we understand all the external 

 influences physical, chemical, and biological to which the or- 

 ganism is subjected. In the case of plants, and still more in 

 that of animals, the conditions of the environment are not 

 constant throughout the life of the individual, but in general 

 it is those experienced in the early stages of development which 

 are most potent in moulding the organism. 



Individual variations, though probably in the main correlated 

 with differences in the environment, may well also result from 

 changes in the internal conditions which are more difficult to 

 analyse. Such variations may be of two kinds. They are 

 qualitative or substantive, when, for instance, they concern the 

 shape or size of the entire plant (Fig. 218) or of any of its parts, 

 the degree of hairiness, the type of colouration (Fig. 219), etc. 

 On the other hand, they are quantitative or meristic, when they 



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