168 DEVELOPMENT AND HEREDITY. 



of development of parts, as distinguished from the 

 period of simple growth in size. 



When the development is modified by a change 

 in the forces of the environment, the change of 

 forces will produce its effect first upon those parts 

 which are the last to perfect their development ; 

 i.e. the parts which, in their developing state, are 

 subjected longest to the action of the new arrange- 

 ment of forces will be the first to succumb to 

 their modifying influence. We find a striking illus- 

 tration of this in the observations of Professor Eimer 

 on the lizard's tail. The tail of the lizard is the 

 last organ to lose its embryonic character. At 

 a time when the other organs are all well developed, 

 the tail still retains at its tip end the neurenteric 

 canal, which joins the central canal of the spinal 

 cord with the cavity of the rudimentary caudal 

 intestine. At the same time the tip of the verte- 

 bral column shows only the embryonic notochord.^ 

 This method of growth, viewed in the light of the 

 present theory, explains the following quotation 

 from Professor Eimer: "The appearance of new 

 characters always takes place at definite parts of 

 the body, usually the posterior end, and during 

 development — with age — passes forwards, while 



1 " Contribution to the Embryology of the Lizard," Henry Orr, 

 yournal of Morphology, 1887. 



