170 THE ORIGIN OF SPECIES 



non facit saltum." This canon, if we look to the present inhab- 

 itants alone of the world, is not strictly correct; but if we include 

 all those of past times, whether known or unknown, it must on 

 this theory be strictly true. 



It is generally acknowledged that all organic beings have been 

 formed on two great laws — Unity of Type, and the Conditions of 

 Existence. By unity of type is meant that fundamental agreement 

 in structure which we see in organic beings of the same class, and 

 which is quite independent of their habits of life. On my theory, 

 unity of type is explained by unity of descent. The expression of 

 conditions of existence, so often insisted on by the illustrious 

 Cuvier, is fully embraced by the principle of natural selection. 

 For natural selection acts by either now adapting the varying 

 parts of each being to its organic and inorganic conditions of life; 

 or by having adapted them during past periods of time: the adap- 

 tations being aided in many cases by the increased use or disuse 

 of parts, being affected by the direct action of the external condi- 

 tions of life, and subjected in all cases to the several laws of growth 

 and variation. Hence, in fact, the law of the Conditions of Exist- 

 ence is the higher law; as it includes, through the inheritance of 

 former variations and adaptations, that of Unity of Type. 



