ADJUSTMENT, DIRECT AND INDIRECT 



attributed to community of origin. No natu- 

 ralist supposes that an annulose animal, or a true 

 mollusk, has ever been developed into a verte- 

 brate. And while the mollusks and vertebrates 

 appear to have diverged from a molluscoid an- 

 cestor akin to the still-living ascidians, the annu- 

 lose sub-kingdom has a totally different pedi- 

 gree. To discover any likeness between the 

 two great groups, we must follow them back to 

 those remotest ancestors who possessed hardly 

 any distinctively animal characteristics. Bearing 

 all this in mind, it is a striking fact that the eye 

 of the cuttle-fish, which is the highest of mol- 

 lusks, appears to be constructed like the eyes 

 of vertebrates. It apparently contains not only 

 a similar retina, but also a lens, the choroid and 

 sclerotic tunics, and the vitreous and aqueous 

 humours. Now this coincidence cannot be due 

 to community of inheritance, for the vertebrate 

 and molluscous sub-kingdoms are linked to- 

 gether only at their lowest extremities, and 

 while the lowest vertebrate has an eye far infe- 

 rior to the one just described, the molluscoid 

 ascidians have merely rudimentary eye-spots. 

 The coincident structures have therefore been 

 independently developed. Again, Mr. Mivart 

 urges that the agreement cannot be explained 

 on the assumption " that the conditions requi- 

 site for effecting vision are so rigid that similar 

 results in all cases must be independently ar- 

 77 



