DESCENT OF MAN FROM SOME LOWER FORM. 143 



known progenitor of the Articulata, many segments ; and 

 the unknown progenitor of flowering plants, many leaves 

 arranged in one or more spires. We have also formerly 

 seen that parts many times repeated are eminently liable 

 to vary, not only in number, but in form. Consequently 

 such parts being already present in considerable numbers, 

 and being highly variable, would naturally afford the 

 materials for adaptation to the most different purposes ; 

 yet they would generally retain, through the force of in- 

 heritance, plain traces of their original or fundamental 

 resemblance. They would retain this resemblance all the 

 more, as the variations, which afforded the basis for their 

 subsequent modification through natural selection, would 

 tend from the first to be similar, the parts being at an 

 early stage of growth alike, and being subjected to nearly 

 the same conditions. Such parts, whether more or less 

 modified, unless their common origin became wholly ob- 

 scured, would be serially homologous. 



DESCENT WITH MODIFICATION THE ONLY EXPLANATION. 



Origin of ^ n wor ^ 3 on natural history, rudimentary 



Species, organs are generally said to have been created 

 page 400. «.f or ^ Q ga j, e Q f symmetry," r in order "to 



complete the scheme of Nature." But this is not an ex- 

 planation, merely a restatement of the fact. Xor is it 

 consistent with itself : thus the boa-constrictor has rudi- 

 ments of hind-limbs and of a pelvis, and if it be said that 

 these bones have been retained " to complete the scheme 

 of Nature," why, as Professor Weismann asks, have they 

 not been retained by other snakes, which do not possess 

 even a vestige of these same bones ? What would be 

 thought of an astronomer who maintained that the satel- 

 lites revolve in elliptic courses round their planets "for 



