August 1, 1898.] 



KNOWLEDGE. 



143 



like mammals (for want of a hotter expression) show a 

 parallel parallelism (to coin another expression) which, 

 if it had continued, mittht have resulted in the develop- 

 ment of a two-clawed ruminant and a two-clawed horse. 

 To our thinking, this is indeed one of the most curious 

 phases of development yet discovered. 



In addition to the parallelism in the evolution of their 

 molar teeth and limbs, the hoofed mammals likewise 

 exhibit the same feature in regard to the second vertebra 

 of the neck. As most of our readers are probably aw-are, 

 the first or atlas vertebra of the neck turns with the 

 head when the latter is moved sideways, the axis of 

 rotation being formed by a process — the odontoid process 

 — arising from the second or axis vertebra, and projecting 

 into the central hollow of the atlas. Now in pigs and 

 likewise all the primitive hoofed mammals, the so-called 

 odontoid process is (as in ourselves) in the form of a 

 flattened peg. On the other hand, in the ruminants, the 

 modern horses, and the camels, which, as we have seen, 

 represent three distinct ]>h\ihi of the order, this peg has, 

 however, become modified into a spout-like half-cylinder, 

 which must clearly have been separately evolved in each 

 of these three groups. It is true that such a half-cylinder 

 affords a far better basis of support for a heavy skull than 

 does a mere peg : but the curious part of the matter is 

 why these half-cylinders should be so exactly alike in the 

 different groups, seeing that as it would not be difficult to 

 design some other structural modification by which the 

 same end might have been attamed, there is no necessity 

 for their similarity. 



Our last example of parallelism will be drawn from two 

 groups of extinct North American hoofed mammals, to which 

 brief allusion has already been made in our article on 

 " Tusks and their Uses " ; the one group being known as 



Skull of Pi'uhiceras. (After O.-Iion 



uintatheres, while the second is represented by a single 

 species to which the name of Pmluci'ms has been applied. 

 Now, although the uintatheres have five-toed feet approxi- 

 mating in structure to those of elephants, whde in I'mto- 

 rcnis each foot approximated to the ruminant type, in both 

 groups the skull, as shown in the accompanying figure, 

 was armed with several pairs of large, irregular, bony pro- 

 cesses, which during life may have been sheathed in horn ; 

 while in each case a pair of long tusks projected from the 

 upper jaws, which were totally devoid of front or incisor 

 teeth. Had such skulls been discovered without any 

 indication as to the nature of the limbs with which they 

 were associated, they would inevitably have been assigned 

 to the same group of animals. The resemblance existing 

 between them is, however, clearly due to parallel develop- 

 ment, and we are thus shown another striking instance of 

 caution necessary in endeavouring to determine the affinities 

 of extinct animals from the evidence of incomplete remains. 

 Our last instance of parallelism has been already alludsd 

 to in an article entitled "The Oldosl Fishes and their Fins," 

 in the course of which it was shown that while both the most 



ancient birds and the oldest fishes had long tapering tails 

 with the joints of the backbone gradually diminishing in 

 size, and each carrying either a pair of feathers or a pair 

 of fin-rays, in all the modern representatives of the former 

 group, and in a large section of the latter, the end of the 

 vertebral column has been aborted into a composite bone, 

 from which either the feathers or the rays of the tail 

 diverge in a fan-like manner. 



In conclusion, we may say that although there is no 

 very great difficulty in satisfactorily accounting for external 

 parallelism obviously due to the necessity for adaptation 

 to a particular mode of life, or in explaining those instances 

 where a particular result has been brought about by 

 different methods, yet when we find precisely similar 

 structural modifications in different groups of animals 

 which clearly cannot be traced to a common ancestry, and 

 for which equally efficient substitutes could be readily 

 suggested, we are fain to confess that the ordinarily 

 accepted explanations of evolution appear to us altogether 

 inadequate. Putting this aspect of the matter aside, our 

 readers will, however, see from the imperfect sketch 

 given above what an important factor in evolution parallel 

 development really is, and how largely it is likely in the 

 future to modify our present views as to the mutual 

 relationships of animated nature. 



GALLS AND THEIR OCCUPANTS.-II. 



By E. A. Butler. 

 {Continued from page 128.) 



NO other English tree is such a favourite with gall- 

 flies as the oak ; nor is this to be wondered at, 

 since the same tree is by far the most popular 

 in the insect world at large. About fifty species 

 of gall-flies are known to attack the two kinds 

 of oak tree which are found wild in this country, and 

 many others are dependent upon the oaks of other lands. 

 No part of the tree is exempt from attack. The unex- 

 panded leaf-buds, the full-grown leaves, the male catkins, 

 the acorns, the young twigs, the leaf-stalks, the bark, and 

 even the roots, all have their special inhabitants, each 

 producing its own characteristic excrescence. Many of 

 these galls are small and insignificant, and hence not 

 generally known ; but in addition to these inconspicuous 

 things, the oak supplies both the conspicuous marble galls 

 already described, and also several other very well-known 

 kinds. One of the most familiar is the '' oak-apple," a 

 soft, exceedingly succulent, somewhat irregularly spheroidal 

 mass of tissue found on the young twigs in the position of 

 a terminal leaf-bud. Its colour is pale yellowish or 

 brownish white, dashed with rosy pink whore the sun has 

 shone most directly upon it. It 

 does not turn hard as the marble 

 gall does, but, though like a juicy 

 apple at first, becomes ultimately 

 a light brown, dry and spongy 

 mass, resembling a small shri- 

 velled apple. When picked in 

 this dried condition it is found 

 to be astonishingly light. 



The oak-apple results from the 

 punctures of the gall-fly named 

 Amln'ciis termiiiiilis. In the 

 centre of its mass it contains a 

 number of hard, oval, woody 

 cells (Fig. i), each the home of a gall-fly larva. It is not 

 therefore like the marble gnll, which in its normal condition 



FiCr. 4. — Sec-fion of Oak-i>.p]ilo, 

 sliowincr cells of Gall-lli' s. 



