THE ARGUMENTS PROM EMBRYOLOGY. 461 



of many stories, let out in flats to different tenants. For the 

 increased passing up and down, a staircase wider at its lower 

 part will be required. If now the builder, when putting up 

 the ground floor, follows the old dimensions, then after all the 

 stories are built, the lower part of the staircase, if it is to 

 yield equal facilities for passage, must be reconstructed. 

 Instead of a staircase adapted to those few stories which the 

 original type of house had, economy will dictate a pre-adapta- 

 tion of the staircase to the additional stories. 



On carrying this idea with us, we shall see that if from 

 some type of organism there is evolved a type in which 

 enlargement of a certain part is needed to meet increased 

 functions, the greater size of this part will begin to show 

 itself during early stages of unfolding. That unbuilding and 

 rebuilding which would be needful were it laid down of its 

 original size, will be made needless if from the beginning 

 it is laid down of a larger size. Hence, in successive genera- 

 tions, the greater prosperity and multiplication of individuals 

 in which this part is at the outset somewhat larger than 

 usual, must eventually establish a marked excess in its 

 development at an early stage. The facts agree with this 

 inference. 



Referring to the contrasts between embryos, Mr. Adam 

 Sedgwick says that " a species is distinct and distinguishable 

 from its allies from the very earliest stages." Whereas, 

 according to the law of von Baer, " animals so closely allied as 

 the fowl and duck would be indistinguishable in the early 

 stages of development," "yet I can distinguish a fowl and 

 a duck embryo on the second day by the inspection of a single 

 transverse section through the trunk." This experience 

 harmonizes with the statement of the late Prof. Agassiz, that 

 in some cases traits characterizing the species appear at an 

 earlier period than traits characterizing the genus. 



Similar in their implications are the facts recently pub- 

 lished by Dr. E. Mehnert, concerning the feet of pentadactyle 

 vertebrates. A leading example is furnished by the foot in 



