292 THE RECAPITULATION THEORY 



} represent more or less closely, in more or less 

 ^modified manner, the successive ancestral stages 

 through which the present condition has been 

 acquired^ Evolution tells us that each animal has 

 had_a__peidigree in the past. Embryology reveals 

 to us this ancestry, because every animal in its 

 own_jiyelopment repeats this history, climbs up 

 itsown_genealogical tree. Such is the Recapitula- 

 3on_Theory J hinted at by Agassiz, and suggested 

 more directly in the writings of von Baer, but first 

 clearly enunciated by Fritz Muller, and since elabo- 

 rated by many, notably by Balfour, and by Ernst 

 HaeckeJ^. It is concerning this theory, which 

 forms the basis of the science of Embryology, and 

 which alone justifies the extraordinary attention 

 this science has received, that I venture to address 

 you this morning. A few illustrations from differ- 

 ent groups of animals will best explain the practical 

 bearings of the theory, and the aid which it affords 

 to the zoologist of to-day, while these will also 

 serve to illustrate certain of the difficulties which 

 have arisen in the attempt to interpret individual 

 development by the light of past history diffi- 

 culties which I propose to consider at greater 

 length. 



A very simple example of recapitulation is 

 afforded by the eyes of the sole, plaice, turbot, and 

 their allies. These " flat fish " have their bodies 

 greatly compressed laterally ; and the two surfaces, 

 really the right and left sides of the animal are, 

 unlike, one being white, or nearly so, and the other 

 coloured. The flat fish has two eyes, but these 



