I20 BIOLOGICAL LECTURES. 



edge, to frame any satisfactory definition of homology on an 

 embryological basis. 



His himself has attempted such a definition in the following 

 words : " From a physiological point of view definite systems 

 or organs are morphologically equivalent when they arise from 

 the same foundation (aus einer gegebenen Anlage) under the 

 same conditions." ^ Such a correspondence constitutes, ac- 

 cording to His, a "complete homology" in Gegenbaur's sense. 

 If, however, the conditions (Formbedingungen) be not iden- 

 tical, then arises a partial correspondence equivalent to the 

 "incomplete homology" of Gegenbaur. 



This definition is obviously true as far as it goes ; but a 

 little consideration of the facts of normal and experimental 

 embryology show that it is so defective as to be practically 

 worthless. For, in the first place, it demands that completely 

 homologous parts shall be identical in normal development, 

 which is by no means always the case (witness the ganglion of 

 Botrylliis in the bud-embryo and in the egg-embryo, or the 

 summer and winter eggs of Cladocera, or the different species 

 of Balanoglossiis or of Pevipatits). A particularly striking case 

 of this kind is that of the crustacean AlpJieiis (as described by 

 Brooks and Herrick), a single species of which has three dif- 

 ferent modes of development in three localities, although the 

 adults do not perceptibly differ, and two of these modes are 

 widely dissimilar, involving the whole character of the meta- 

 morphosis. It seems impossible to explain this case except 

 under the view that the differences of development result 

 from corresponding differences in the surrounding conditions. 

 In the second place, the conditions of development at par- 

 ticular stages may be artificially altered (so that, for instance, 

 an Qg% is compelled to undergo a mode of cleavage totally 

 unlike the normal) without in the least degree altering the 

 final outcome of the ontogeny. 



Such cases make it certain that changed conditions may 

 profoundly alter the mode of development without perceptibly 

 affecting the end-result (though in many cases the end-result 

 is affected also, as in the case of polymorphic insects, etc.). 



1 Arch. Anat. u. Phys., 1887, p. 43S. 



