34 THE MAMMALIA [SECT. A 



rate; also it is claimed that evolution is discontinuous; again the urgency for 

 an extended study of the characters called determinants (Cuenot) is clearly 

 indicated. 



The second objection is scarcely less important, and it relates to the 

 interpretation placed upon intermediate forms. The usefulness of these 

 examples in suggesting and explaining the general trend of evolution is 

 unquestioned. They fill what otherwise would he gaps, and they reduce the 

 extent of gradations. But it is clear from the first objection that inter- 

 mediate forms need not constitute the actual line of descent ; it is certain 

 that many of them are not "ancestral," and a warning must be given against 

 the common tendency to regard them in that light. But even those to which 

 the term "ancestral" may not be applied, existed or do exist, and their 

 position and relations demand elucidation. This is a very serious matter. 

 For it is by some alleged (cf. Galton, op. cit. p. 33) that at least in certain 

 instances, intermediate forms are to be regarded as "unstable varieties, whose 

 descendants had reverted : they might be looked upon as tentative and falter- 

 ing steps, taken along parallel courses of evolution, and afterwards retraced." 

 If we add that the descendants might have also died out, and that some of 

 the parallel courses of evolution had come to an end, then the foregoing 

 projjosition need only to be stated to compel acceptance. Evidently the diffi- 

 culty remains that many intermediate forms of the highest interest are 

 represented by only the most scanty fragments. To determine from these 

 whether their possessor was in the ancestral line, or alternatively on a 

 side-track, has provided matter for many an animated discussion, yet the 

 difficulty is too often ignored. Its solution or removal is of fundamental 

 importance in regard to such instances as the fossil remains from Trinil 

 (Pithecanthropus), Mauer (H. heidelbergensis), and Piltdown (Eoanthropus). 

 Returning to the general question of Evolution, it may be repeated that, 

 even with such qualifications, the general analogy of a sequence or chain 

 is still perfectly justifiable. 



It should be further remembered that though in an uniform 

 linear chain all the links are of equal value, yet in the variable 

 series of animals known to us, we may chance to find isolated links 

 of very different significance, the difference depending on the 

 groups of animals connected by the link, whether the latter be 

 known in the fossil or recent state. Moreover the metaphor of 

 a linear chain is not so exact as that of a sheet of chain-armour 

 in which a single link may bring three or four other links into 

 mutual relation. 



To take some examples, there may be cited such animals as 

 the Archaeopteryx, a form which suggests a link between the 

 Glass of Birds and the Class Reptilia : the Galeopithecus volans, 



