328 Heredity. 



gemmules and the consequent variations are due to the 

 direct action of changed conditions upon certain cells of 

 the body, and any change which affects all the individuals 

 of a species will cause the same part to vary in all of them 

 at the same time. This objection to the law of natural 

 selection is thus entirely removed. 



The evolution of a complicated organism, or the modi- 

 fication of any part which includes a number of compli- 

 cated structures, without destroying their harmonious 

 adjustment to each other, demands a very great number 

 of variations, and if these are fortuitous, we may well 

 doubt whether there has been time enough for the evolu- 

 tion of life by natural selection. According to our theory 

 of heredity, a change in one part of the body is in itself 

 a cause of variation in related parts; and as changes thus 

 tend to occur where and when they are needed, the time 

 which is demanded for the evolution of a complicated 

 organ by natural selection is brought within reasonable 

 limits, and one of the most fundamental objections is 

 thus completely removed. 



There are many reasons for believing that variations 

 under nature may not be so minute as Darwin supposes, 

 but that evolution may take place by jumps or saltations. 

 According to our view a change in one part will disturb 

 the harmony of related parts, and will cause their cells to 

 throw off gemmules. A slight change in one generation 

 may thus become in following generations a very con- 

 siderable modification, and there is no reason why natural 

 selection should not be occasionally presented with great 

 and important saltations. 



The law of heredity also enables us to understand the 

 occasional occurrence of parallel or analogous variation, 

 and the parallel evolution of polyphylletic-groups. 



