GREAT STEPS IN ORGANIC EVOLUTION 395 



there were in this new type of nervous system fresh possi- 

 bilities of elaborate and subtle integration, of registering 

 experience and experiments on a large scale, yet without 

 interfering with openness of mind, and probably of a richer 

 and freer stream of inner life. It was in Vertebrates first 

 that bone made its appearance, and formed a living internal 

 skeleton pervading the whole body. This contributed not 

 a little to integration. In the establishment of numerous 

 glands of internal secretion, whose hormones or regulative 

 substances are distributed throughout the body, a chemical 

 integration began to operate or to operate on a larger scale 

 (for we know very little of such organs of internal secre- 

 tion in backboneless animals). It is difficult to exaggerate 

 the importance of these organs in backboned animals, for 

 they are regulatory arrangements which secure smooth work- 

 ing. In one way they make the organism more automatic; 

 in another way they set it free for higher issues. How 

 much is our peace of mind dependent on the insignificant 

 looking speed-regulator, which we call the thyroid gland, or 

 on what the adrenals do in the way of rapidly altering the 

 blood-pressure. 



Skulls began with the hags and lampreys — simple gristly 

 brain-boxes to start with; jaws and paired fins, scales and 

 typical gills with the true fishes; digits, true lungs, vocal 

 cords, and a mobile tongue — what a list of acquisitions — 

 with the phlegmatic amphibians; the ante-natal robes (or 

 foetal membranes) known as amnion and allantois w^ith the 

 reptiles; a four-chambered heart with the crocodilians; warm- 

 bloodedness or keeping the temperature of the body approxi- 

 mately constant with birds and mammals, which also show 

 an enormous advance in brain development, — the big-brain 

 type at length coming to its own. The usually prolonged 



