ANIMAL PEDIGREES 243 



optical properties of the eye, and will be a gain, as 

 it will prevent the entrance of foreign bodies into 

 the cavity of the eye. The formation of the lens 

 by deposit of cuticle is the next step The gain 

 here is increased distinctness and increased bright- 

 ness of the image, for the lens will focus the rays 

 of light more sharply on the retina, and will allow 

 a greater quantity of light, a larger pencil of rays 

 from each part of the object, to reach the corre- 

 sponding part of the retina. The eye is now in the 

 condition in which it remains throughout life in the 

 snail and other gastropods. Finally the formation 

 of the folds of skin known as iris and eyelids 

 provides for the better protection of the eye, and 

 is a clear advance on the somewhat clumsy method 

 of withdrawal seen in the snail, 



It is not always possible to point out so clearly 

 as in the above instance the particular advantage 

 gained at each step, even when a complete develop- 

 mental series is known to us ; but in such cases, as 

 for instance in Orbitolites, our difficulties may be 

 largely ascribed to ignorance of the particular con- 

 ditions that confer advantage in the struggle for 

 existence, in the case of the forms we are dealing 

 with. That ontogeny really is a repetition of phylo- 

 geny must I think be admitted, in spite of the 

 numerous and various ways in which the ances- 

 tral history may be distorted during the actual 

 development. 



Before leaving the subject, it is worth while 

 inquiring whether any explanation can be found 

 of recapitulation. A complete answer can certainly 



