PRESIDENTIAL ADDRESS. 45 



Oa this view the actual development of any animal is strictly limited 

 at both ends : it must commence as an egg, and it must end in the 

 likeness of the parent. The problem of recapitulation becomes 

 thereby greatly narrowed ; all that remains being to explain why the 

 intermediate stages in the actual development should repeat the 

 intermediate stages of the ancestral history. 



Although narrowed in this way, the problem still remains one of 

 extreme difficulty. 



It is a consequence of the Theory of Natural Selection that identity 

 of structure involves community of descent : a given result can only 

 be arrived at through a given sequence of events : the same morpho- 

 logical goal cannot be reached by two independent paths. A negro 

 and a white man have had common ancestors in the past ; and it is 

 through the long-continued action of selection and environment that 

 the two types have been gradually evolved. You cannot turn a white 

 man into a negro merely by sending him to live in Africa : to create a 

 negro the whole ancestral history would have to be repeated ; and it 

 may be that it is for the same reason that the embryo must repeat 

 or recapitulate its ancestral history in order to reach the adult goal. 



I am not sure that we can at present get much further ; but the 

 above considerations give opportunity for brief notice of what is 

 perhaps the most noteworthy of recent embryological papers, Kleinen- 

 berg's remarkable monograph on Lopadorhynchus. 



Kleinenberg directs special attention to what is known to evolu- 

 tionists as the difficulty with regard to the origin of new organs, 

 which is to the effect that although natural selection is competent to 

 account for any amount of modification in an organ after it has 

 attained a certain size, and become of functional importance, yet that 

 it cannot account for the earliest stages in the formation of an organ 

 before it has become large enough or sufficiently developed to be of 

 real use. The difficulty is a serious one ; it is carefully considered by 

 Mr. Darwin, and met completely in certain cases ; but as Kleinenberg 

 correctly states, no general explanation has been offered with regard 

 to such instances. 



As such general explanation Kleinberg proposes his theory of the 

 development of organs by substitution. He points out that any 

 modification of an organ or tissue must involve modification, at least 



