Complex Life — The Invertebrates 



where some part of the foot is subjected to constant rubbing, 

 Nature thickens the skin of the disturbed part until it 

 becomes thicker and calloused as a means of resisting the 

 friction and as a protection to the more tender parts. 



In like or similar manner, little appendages began to 

 form on the bodies of the fishes. These developed with 

 use and as their constant use continued, Nature developed 

 therein bones, supporting muscles and all that goes to make 

 a useful leg. Scales were cast off and a flexible skin then 

 covered the entire body. With the development of legs 

 and other body changes, the new animal could then hop or 

 crawl on land. Thus, through adaptations, another great 

 step in the chain of life was taken and another class, that of 

 the amphibia, came into being. 



Nature does not abandon the work in which she has been 

 engaged for millions of years in a single day. While the 

 new class of animals took on new habits of life on the land, 

 they still clung to many of the habits of their aquatic ances- 

 tors. Amphibians, even to the present time, are essentially 

 fishes in their development in early life. The amphibians 

 and in fact every animal that is hatched from an egg repeat 

 the history of their ancestral development. Tadpoles are 

 hatched in the water. When they are first hatched, they 

 are blind and mouthless and have neither ears, nose nor 

 gills. In the eggs from which they have come, they have 

 repeated all the life processes that have preceded them. But 

 soon after hatching, they develop a mouth, eyes, ears, gills, 

 nose and a horny jaw, but they are, as yet, essentially fishes. 

 They are not yet ready for life on land but within a few 

 weeks hind limbs begin to develop beneath the skin and, like 



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