i 4 2 MODERN IDEAS OF EVOLUTION 



present day? Haeckel s answer is a curious example 

 of his method 



The great significance of the five digits depends on the 

 fact that this number has been transmitted from the am 

 phibia to all higher vertebrates. It would be impossible to 

 discover any reason why in the lowest amphibia, as well as 

 in reptiles and in higher vertebrates up to man, there should 

 always originally be five digits on each of the anterior and 

 posterior limbs, if we denied that heredity from a common 

 five-fingered parent form is the efficient cause of this pheno 

 menon ; heredity can alone account for it. In many am 

 phibia certainly, as well as in many higher vertebrates, we 

 find less than five digits. But in all these cases it can be 

 shown that separate digits have retrograded, and have 

 finally been completely lost. The causes which affected 

 the development of the five-fingered foot of the higher ver 

 tebrates in this amphibian form from the many-fingered 

 foot (or properly fin) must certainly be found in the adapta 

 tion to the totally altered functions which the limbs had to 

 discharge during the transition from an exclusively aquatic 

 life to one which was partially terrestrial. While the many- 

 fingered fins of the fish had previously served almost exclu 

 sively to propel the body through the water, they had now 

 also to afford support to the animal when creeping on the 

 land. This effected a modification both of the skeleton and 

 of the muscles of the limbs. The number of fin-rays was 

 gradually lessened, and was finally reduced to five. These 

 five remaining rays were, however, developed more vigo 

 rously. The soft cartilaginous rays became hard bones ; the 

 rest of the skeleton also became considerably more firm ; 

 the movements of the body became not only more vigorous 

 but also more varied 



and the paragraph proceeds to state other ameliora 

 tions of muscular and nervous system supposed to be 



