The Recapitulation Theory in Biology 33 



generalized, and that they remain up till now in the state of 

 exceptional facts." 61 



The second mode of answering the question is that of studying 

 the development of fossils themselves, and this has been re- 

 ported in the earlier section dealing with the laws of accelera- 

 tion and retardation. The evidence for recapitulation there 

 presented belongs in character with that just cited. 



With the exception of this last group of facts relating to the 

 shells of invertebrates the authorities quoted are in general 

 agreement. We must conclude that the palaeontological evi- 

 dence for recapitulation is not impressive. Among certain 

 groups of animals in some part of their history, some forms 

 are embryonic in a broad way, and a considerable number of 

 particular instances of the embryonic or youthful retention 

 of ancestral adult traces are noted. Apparently the conditions 

 favoring recapitulation in fossil shells are peculiar to such char- 

 acters. 



(b) The Facts of Embryonic Resemblance. 



The historical importance of von Baer's law for the present 

 topic has been obvious enough. Darwin, Haeckel, and Spen- 

 cer either made it the point of departure of their arguments or 

 drew heavily upon it. Spencer showed clearly in the words 

 quoted from him that a consistent evolutionary application of 

 the law is equivalent to recapitulation. But the limitations 

 of the formula were noticed very early. Darwin alluded to the 

 common, but not invariable, resemblance between embryos in 

 the same class. Fritz Miiller speaks of " developmental forms 

 now so totally different in the nearest allies, now so surpris- 

 ingly similar in members of the most distant groups." 52 Spen- 

 cer regarded the generalization as an average truth only. 

 Haeckel and others found great difficulty in reducing to an order 

 consistent with the law the observed differences between com- 

 parable embryonic stages. The following extracts will indicate 

 the attitude of more recent writers toward the facts in question: 



"Von Baer's statement appears to be erroneous from a modern 

 point of view in the following respects. We know that in certain 

 large groups some forms develop in a very different way from 



"pp. 254-256. 



" For Darwin, p. 97. 



