172 VESTIGES OF THE 



is perfect ; nor may even tliese small discrepancies appear 

 Avhen the order of fossils shall have been further in- 

 vestigated, or a moi'e correct scale shall have been formed. 

 Meanwhile, it is a wonderful evidence in favour of our 

 hypothesis, that a scale formed so arbitrarily should 

 coincide to such a nearness with our present knowledge 

 of the succession of animal forms upon earth, and also 

 that both of these series should harmonise so well with 

 the view given by modern physiologists of the embryotic 

 progress of one of the organs of the highest order of 

 animals. 



The reader has seen physical conditions several times 

 referred to, as to be presumed to have in some way 

 governed the progress of the develoi^ment of the zoo- 

 logical circle. This language may seem vague, and, it 

 may be asked — can any particular physical condition be 

 adduced as likely to have affected development ? To this 

 it may be answered, that air and light are probably 

 amongst the principal agencies of this kind which 

 operated in educing the various forms of being. Light 

 is found to be essential to the development of the 

 individual embryo. When tadpoles were placed in a 

 perforated box, and that box sunk in the Seine, light 

 being the only condition thus abstracted, they grew to 

 a great size in their original form, but did not pass 

 through the usual metamorphose which brings them to 

 their mature state as frogs. The proteus, an animal of 

 the frog kind, which never acquires perfect lungs so as 

 to become a land animal, is presumed to be an example 

 of arrested development, from the same cause. When, 

 in connexion with these facts, we learn that human 

 mothers living in dark and close cells under ground — 

 that is to say, with an inadequate provision of air and 

 light — are found to produce an unusual proportion of 



