500 



THE CHANGING WORLD 



makes more probable its relationship to spiders and scorpions, a 

 conclusion which is confirmed physiologically by blood tests, as will 

 be pointed out later. 



The microscopic water flea Daphnia, the sedentary rock barnacle 

 Balanus, that according to Huxley "stands on its head and kicks 



food into its mouth with 

 its legs," the amorphous 

 parasitic degenerate 

 lump Sacculina, some- 

 times infesting the ab- 

 domen of crabs, and the 

 familiar free-swimming 

 lobster Homarus, are very 

 diverse in adult appear- 

 ance. No one would 

 ordinarily suspect them 

 of being related, yet an 

 examination of their em- 

 bryonic history reveals 

 unmistakably that they 

 are crustacean cousins 

 all of one blood. Thus 

 does embryology, the sci- 

 ence concerned with the 

 development of the indi- 

 vidual, furnish evidence of relationship that otherwise may not at 

 once be apparent. 



The everyday miracle of adult organisms developing from eggs or 

 seeds loses much of its force because of its very familiarity. We 

 cease to wonder that a chick can hatch out of a hen's egg, or that a 

 gorgeous flower can arise from a seed buried in the ground, because 

 its repeated occurrence comes within the span of our everyday experi- 

 ence. If we could live and observe for a million years, no doubt the 

 panorama of evolution would become as obvious, and be as unques- 

 tionably acceptable, as that of individual development. 



The fact that our horizon is limited by a span of " three score years 

 and ten" foreshortens our vision so that we lose the perspective 

 needed to make the picture clear and in focus. The limitations of 

 human life are in this respect a decided handicap to a more complete 

 understanding of evolutionary processes. 



These four extremely unlike animals, fitted for 

 quite different careers, all have in common the 

 heritage of the crustacean plan. 



