DIFFICULTIES ON THEORY 177 



another or in charming- tbe females. Moreover when 

 a modification of structure has primarily arisen from 

 the above or other unknown causes, it may at first 

 have been of no advantage to the species, but may 

 subsequently have been taken advantage of by the de- 

 scendants of the species under new conditions of life 

 and with newly acquired habits. 



To give a few instances to illustrate these latter 

 remarks. If green woodpeckers alone had existed, 

 and we did not know that there were many black and 

 pied kinds, I dare say that we should have thought 

 that the green colour was a beautiful adaptation to 

 hide this tree-frequenting bird from its enemies ; and 

 consequently that it was a character of importance and 

 might have been acquired through natural selection ; 

 as it is, I have no doubt that the colour is due to some 

 quite distinct cause, probably to sexual selection. A 

 trailing bamboo in the Malay Archipelago climbs the 

 loftiest trees by the aid of exquisitely constructed 

 hooks clustered around the ends of the branches, and 

 this contrivance, no doubt, is of the highest service to 

 the plant ; but as we see nearly similar hooks on many 

 trees which are not climbers, the hooks on the bamboo 

 may have arisen from unknown laws of growth, and 

 have been subsequently taken advantage of by the 

 plant undergoing further modification and becoming a 

 climber. The naked skin on the head of a vulture is 

 generally looked at as a direct adaptation for wallowing 

 in putridity ; and so it may be, or it may possibly be 

 due to the direct action of putrid matter ; but we 

 should be very cautious in drawing any such inference, 

 when we see that the skin on the head of the clean- 

 feeding male turkey is likewise naked. The sutures 

 in the skulls of young mammals have been advanced 

 as a beautiful adaptation for aiding parturition, and 

 no doubt they facilitate, or may be indispensable 

 for this act ; but as sutures occur in the skulls of 

 young birds and reptiles, which have only to escape 

 from a broken egg, we may infer that this structure 

 has arisen from the laws of growth, and has been 



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