domingo, agosto 10, 2008
Advertencias de Darwin sobre selección natural
The Origin of Species by Means of Natural Selection Or, The Preservation of Favoured Races in the Struggle for Life By Charles Darwin: "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 conceal this tree frequenting bird from its enemies and consequently that it was a character of importance and had been acquired through natural selection. As it is the colour is probably in chief part due to sexual selection. A trailing palm 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 and which as there is reason to believe from the distribution of the thorn bearing species in Africa and South America serve as a defence against browsing quadrupeds so the spikes on the palm may at first have been developed for this object and subsequently have been improved and taken advantage of by the plant as it underwent further modification and became a climber. The naked skinhead of a vulture is generally considered 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 taken advantage of in the parturition of the higher animals""