ADDRESS TO THE BRITISH ASSOCIATION. 699 



theory, the present order of the organic world has been caused 

 by the action of two laws, known as the laws of heredity and of 

 variation. The law of heredity is familiarly exemplified by the 

 well-known fact that offspring resemble their parents. Not only, 

 however, do the offspring belong to the same species as their 

 parents, but they inherit the individual peculiarities of their 

 parents. It is on this that the breeders of cattle depend, and it 

 is a fact of every-day experience amongst ourselves. A further 

 point with reference to heredity to which I must call your atten- 

 tion is the fact that the characters, which display themselves at 

 some special period in the life of the parent, are acquired by the 

 offspring at a corresponding period. Thus, in many birds the 

 males have a special plumage in the adult state. The male 

 offspring is not, however, born with the adult plumage, but only 

 acquires it when it becomes adult. 



The law of variation is in a certain sense opposed to the law 

 of heredity. It asserts that the resemblance which offspring 

 bear to their parents is never exact. The contradiction between 

 the two laws is only apparent. All variations and modifications 

 in an organism are directly or indirectly due to its environments; 

 that is to say, they are either produced by some direct influence 

 acting upon the organism itself, or by some more subtle and 

 mysterious action on its parents; and the law of heredity really 

 asserts that the offspring and parent would resemble each other 

 if their environments were the same. Since, however, this is 

 never the case, the offspring always differ to some extent from 

 the parents. Now, according to the law of heredity, every ac- 

 quired variation tends to be inherited, so that, by a summation 

 of small changes, the animals may come to differ from their 

 parent stock to an indefinite extent. 



We are now in a position to follow out the consequences of 

 these two laws in their bearing on development. Their applica- 

 tion will best be made apparent by taking a concrete example. 

 Let us suppose a spot on the surface of some very simple organ- 

 ism to become, at a certain period of life, pigmented, and there- 

 fore to be especially sensitive to light. In the offspring of this 

 form, the pigment-spot will reappear at a corresponding period ; 

 and there will therefore be a period in the life of the offspring 

 during which there is no pigment-spot, and a second period in 



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