46 THE EVOLUTION OF MAN. 



on the same plan. A third, the vascular system, now appears, 

 which is certainly sufficiently similar to the two earlier 

 structures to allow of its form being easily recognized as 

 that which has been described as approximately common to 

 all the systems. The fourth system, the intestinal canal, now 

 follows ; this, again, is formed on the same plan, and, when 

 completed and closed, resembles the three earlier systems." 

 In this most important discovery Wolff laid the first 

 foundations of the fundamental " germ-layer theory " which 

 was not completely developed till long afterwards, by 

 Pander (1817) and by Baer (1828). It is true that Wolff's 

 propositions are verbally incorrect, but in them he reached 

 the truth as nearly as was then possible, and as was to be 

 expected. We shall presently see how nearly he approached 

 to the real state of the case. 



Wolff owes much of his comprehensive conception of 

 nature to the fact, that he was as good a botanist as a 

 zoologist. He studied the history of the development of 

 plants also, and in the field of botany first founded the 

 theory which Goethe afterwards developed in his brilliant 

 treatise on the " Metamorphosis of Plants." Wolff was the 

 first to show that all the various parts of plants may be 

 traced back to the leaf as their common rudiment, or 

 ' fundamental organ." Flower and fruit, with all their 

 parts, consist only of modified leaves. This discovery must 

 have seemed all the more surprising to Wolff, from the fact 

 that he had discovered a simple leaf-like rudiment to be 

 the first form of the embryonic body of animals, as it is of 

 plants. 



We therefore find in Wolff distinct traces of those 

 theories of which, at a much later period, other gifted 



