XXIV.] NATURAL SELECTION. 321 



number of co-operating parts, and the closest co-operation 

 between the parts.-^ It is to be observed, that the fore- 

 going argument is greatly understated. I have spoken only Co-opera- 

 of the co-operation of parts in one oi^gan. But in nature p°rts°in an 

 we have also to do with the co-operation of organs in one organ, and 

 organism ; and most organs are complex ; so that there i^ ^if 

 must be co-operation of many organs, each of them con- organism, 

 sisting of many parts. In some cases, no doubt, the prin- 

 ciple of self-adaptation will account for one part becoming 

 adapted to the rest, as we have seen in speaking of the 

 deer's horns and the bat's wing-muscles ; but this will not 

 be always the case. What greatly increases the difficulty The eye 

 of supposing that the eye can have been formed by natural fo^j^med^on 

 selection is, that it has been formed not on one, but on three 

 three distinct lines of descent. Well-developed eyes are unes of 

 found in the higher orders of Annulosa, MoUusca, and descent. 

 Vertebrata. We need not now discuss the question, 

 whether these three groups are descended from a common 

 ancestor ; if they are, that ancestor must have been, in all 

 probability, of too low an organization to have eyes. We 

 may infer this from the facts that the lowest known ver- 

 tebrate, the amphioxus, has only rudimentary eyes,- and 

 the lower Mollusca and Annulosa have none. The eye 

 must consequently have been separately perfected in those 

 three groups. It is a fact of the same class, that the Skulls of 

 Cephalopoda (cuttle-fish and nautilus), which are the ^^^^ ^ °^ 

 highest of the Mollusca, resemble the Vertebrata not only of Verte- 

 in the general higher development of the nervous system 

 and of the eyes, but in the special and very remarkable 



* H. Spencer has given the emphasis to this argument that it deserves 

 (see, the extract above). Yet he says, though without oflferiug any proof, 

 that the complexities of the sensory organs must be the result of natural 

 selection. I cannot avoid thinking that he has been biassed by a deter- 

 mination to assign a physical cause, such as either self-adaptation or 

 natural selection, for all biological facts, to the exclusion of intelligence. 

 (See his Principles of Biology, vol. u. p. 307.) 



2 " An eye, such as exists in the fish called the lancelet (amphioxus), 

 ■which is so simple that it consists only of a fold -like sac of skin, lined with 

 pigment and furnished with a nerve, but destitute of any other apparatus, 

 being merely covered by transparent membrane." (Darwin's Origin of 

 Species, p. 218.) 



Y 



