58 THE GENESIS OF SPECIES. [CHAP. 



increases and assumes the form of a sac, which changes 

 into the aqueous humour and lens. An outgrowth of 

 brain substance, on the other hand, forms the retina, while 

 a third process is a lateral ingrowth of connective tissue, 

 which afterwards changes into the vitreous humour of 

 the eye. 



The internal ear is formed by an involution of the 

 integument, and not by an outgrowth of the brain. But 

 tissue, in connexion with the latter, becomes in part 

 changed, thus forming the auditory nerve, which places 

 the tegumentary sac in direct communication with the 

 brain itself. 



Now, these complex and simultaneous co -ordinations 

 could never have been produced by insignificant begin- 

 nings, since, until so far developed as to effect the requisite 

 junctions, they are useless. But the eye and ear when fully 

 developed present conditions which are hopelessly difficult 

 to reconcile with the mere action of " Natural Selection." 

 The difficulties with regard to the eye have been well put 

 by Mr. Murphy, especially that of the concordant result 

 of visual development springing from different starting- 

 points and continued on by independent roads. 



He says, 1 speaking of the beautiful structure of the 

 perfect eye, "The higher the organization, whether of an 

 entire organism or of a single organ, the greater is the 

 number of the parts that co-operate, and the more perfect 

 is their co-operation ; and consequently, the more necessity 

 there is for corresponding variations to take place in all 

 the co-operating parts at once, and the more useless will 

 be any variation whatever unless it is accompanied by 

 corresponding variations in the co-operating parts ; while 



1 " Habit and Intelligence," vol. i. p. 319. 



