FISHES 171 



very great importance to it. It is believed to complete 

 our geological discoveriea Just as the various epochs 

 in the earth's growth show first lower then higher 

 organisms, so we find the same gradation once more, in 

 a condensed form, recapitulated in the embryonic 

 development of every animal. 



We shall deal more fully with the significance of the 

 biogenetic law in the seventh chapter. Here we will 

 only point out by means of it why rudimentary organs 

 appear and disappear in the evolution of animals. This 

 we find to be the case. The human ovum needs nine 

 months for its development. In the fourth week it has 

 made considerable progress, but is very unlike a human 

 being, and much more like an animal. It clearly shows 

 slits like the gill-clefts of the fish on each side, and has 

 a striking resemblance to the fish in the arrangement 

 of the heart, the blood-vessels, and the bony skeleton. 

 The clefts are of no use whatever to the unborn child, 

 and they soon disappear again. We may confidently 

 say that there is no stronger proof of man's descent 

 from the fish than these gill-clefts, which are still 

 retained in the descendants of the fish — men — in 

 virtue of the law of heredity, although they have 

 had no purpose or use for millions of years, and they 

 disappear in the further course of development. 



Thus we have got back to the fishes from which we 

 started. They are not only of importance as the lowest 

 class of vertebrates on account of their structure, but 

 their life gives us an interesting illustration of the 

 working of natural selection. The struggle for life is 

 found amongst them in unsparing form. Except the 



