xv PHILOSOPHY OF ZOOLOGY 627 



considering this question perhaps the most important point to 

 attend to is that the process is one of cell-division (with its atten- 

 dant changes in the nucleus) and that the result of it is the 

 formation of three or four cells, one the ovum very much 

 larger than the rest. The polar globules may thus very well be 

 regarded as abortive ova. This, however, is not in itself a suf- 

 ficient explanation of their occurrence in all classes of the Metazoa 

 from the lowest to the highest. Mere vestiges are hardly likely 

 to be so persistent ; so that we should be justified in concluding 

 that the polar globules are abortive ova which persist in all classes 

 of animals because of some function that they perform in prepara- 

 tion for development probably in removing material no longer 

 required, or calculated to retard impregnation and development. 



If, as is admitted by many writers at the present day, natural 

 selection the selection of intrinsic variations be inadequate to 

 explain many of the facts of evolution, there is no alternative but 

 the view that development is partly caused by the transmission of 

 changes brought about in the organism as a result of its own 

 activity, directed and conditioned by the environment, and of the 

 action of external agencies. By what means such changes can 

 become impressed on the germinal substance it is difficult to 

 understand. But, as already pointed out, a centrifugal influence 

 of the reproductive cells on the development of distant parts is 

 established by numerous instances, and the mode of transmission 

 of the influence is as difficult to conceive in the one case as in the 

 other. 



s s 2 



