CHAPTER V 



FISHES 



Origin of terrestrial vertebrates and of lungs. Similarities in the 

 structure of animals. Transformation of organs. Creation 

 or evolution ? Many animals are worse off than others. 

 Selection only creates what is necessary. Atrophy of useless 

 organs. Rudimentary organs in man. Degeneration of organs 

 by panmixis. Indifferent characteristics of animals. The 

 differences between species are adaptations. Correlation. 

 Animals that are beyond the range of selection. Qualities and 

 quantities. Explanation of atrophied organs by economy of 

 sustenance and negative selection. Impossible to explain many 

 rudimentary organs. The biogenetic law. Gills in the human 

 embryo. Predatory fishes. The rhodeus and the pond-mussel. 

 Senses of fishes, their dangers. History of the eel and the 

 salmon. Artificial selection of fishes. 



As we make our way down from the sunny heights of 

 the forest-crowned hills to the green valley, we often 

 halt for a moment, when our eye falls on the brook by 

 the way, and catches sight of a trout. We admire the 

 ease and restfulness with which the animal meets the 

 rush of the water over the stones. 



But there is something more wonderful in the power 

 of the fish to penetrate into the depths of the sea, and 

 remain there without any exertion of the muscles. As 

 a fact, the fish can halt at any dep^h without moving a 

 fin ; it has a special organ that enables it to do this. 



The swimming-bladder is the name of this organ, and 



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