PLANT AND ANIMAL PROGRESSION §1 



In the vertebrates, nature solves the vexing problem of 

 body support by inventing an internal, living skeleton 

 which can grow with the animal and can be articulated into 

 greater flexibility than any previously known. The inver- 

 tebrate skeleton, as in the insect, is a dead outside structure; 

 it is not flexible in growth and sets up restrictions in the life 

 cycle. In primitive forerunners of the vertebrates, like the 

 lamprey, the skeleton is a long rod of stiff tissue running 

 almost the full length of the body. It is this rod-like struc- 

 ture (notochord) which, in later vertebrates, is replaced by 

 the backbone. In all the embryos of vertebrates, including 

 man, the notochord is formed before the true skeleton ap- 

 pears, a repetition of an ancestral condition. Basically, the 

 vertebrates take over the segmentation of the annelid-arth- 

 ropod line, the tube-within-a-tube body-plan, the primitive 

 kidney system which they at once improve upon in an 

 ascending series, and bilateral symmetry, besides embryonic 

 stages such as the blastula and gastrula. 



A more or less new characteristic of the vertebrates is the 

 development and arrangement of gills for taking oxygen 

 out of the water. Gills are highly eflicient devices and so 

 much a part of the race that, even in the land forms like 

 man, they are elaborately repeated in the embryo only to 

 be torn down later and made over into other structures such 

 as the bones of the middle ear, cartilage for the windpipe, 

 arteries to the brain, and so forth. 



Another basic characteristic is a dorsal, tubular nerve 

 cord, the structure from which the vertebrates develop their 

 incomparable nervous system. Almost at once the improve- 

 ments in the nervous system appear. Here in the evolution 

 of the brain is a still inexhaustible source of progress. Early 

 in the ancestry of fish two centers of correlation were set 

 up, one for sensory knowledge, the other for the coordina- 

 tion of motor action; and the two centers were united by 

 nerve connections. Present-day fish are at this stage. Then 

 the evolution of a new region begins, the cerebral hemi- 



