AuiiusT 1, 1893.] 



KNOWLEDGE. 



141 



■■■/, 



X'- 



^^ A N ILLUST RATED "^^ 



MAGAZINE OF SCIENCE 



SIMPLY WORDED— EXACTLY DESCRIBED 



LONDON: AUGUST 1, 1893. 



CONTENTS. 



Parallelism In Development. B_v R. Ltbekker, 



B.A.Cantab 



Galls and their Occupants — II. By E. A. Btjtler 



The Sun as a Bright-Line Star. By Mi.ss A. M. Cleuke 



Science Notes 



The Great Lunar Grater Tycho. By A. C. Rant.\rd ... 

 Letters :-,T. E. Holt; AV. T. Lvnn ; "W. II. S. Monck 



Notices of Books , ... 



The Migration of Birds. By ft. T\". Briictx, MA., B.Sc. 



The Face of the Sky for August. By Hebbeet Sadler, 

 F.R.A.S 



Chess Column. By C. D. Locock, B.A.Oxon 



PAGE 

 141 



143 

 146 

 14S 

 149 

 153 

 155 

 155 



158 

 159 



PARALLELISM IN DEVELOPMENT. 



By R. Lydekkeb, B.A.Cantab. 



IN the course of our three preceding articles in 

 Knowledge it has been to a great extent our aim 

 to show that certain animals may resemble one 

 another very closely in general external appearance, 

 or may possess certain peculiar structural features 

 m common, without being in any way intimately related ; 

 thus rendering it evident that such similarities of form or 

 structure have been independently acquired, and are not 

 inherited from a common ancestor. It has been shown, 

 for instance, that mammals belonging to several distinct 

 orders, or to different families of the same order, may 

 assume such a marked external resemblance to the 

 common mole as to be designated in popular langua<Te by 

 the same general title ; while in other cases a more or less 

 striking approximation to the type of the ordinary 

 hedgehog has resulted from the independent development 

 of very similar spines in totally distinct groups. Further- 

 more, it has been pointed out in the third of the three 

 articles that large tusks of very similar form may be 

 independently developed in the jaws of totally different 

 distinct groups of mammals ; and even that certain 

 extinct reptiles have acquired tusks which are almost 

 indistinguishable from those of some of the carnivorous 

 mammals, there being no direct relationship between the 

 members of these groups. 



With regard to the external similarities of form in the 



above-mentioned instances, we have seen reason to 

 believe that their inducing cause has been either 

 similarity of habit or the need of protection ; while in 

 the case of the tusks the similarity may in certain 

 instances be due to the necessity for efficient offensive 

 weapons. Be their causes, however, what they may, it is 

 evident that such resemblances among animals, which are, 

 so to speak, accidental, indicate wliat may be termed a 

 kind of parallel development ; and as such parallelism in 

 development, or shortly " parallelism," has recently 

 attracted a good deal of attention among biologists, we 

 propose in this communication to present to our readers 

 some of the more striking instances of such similarity. 

 In the instances above alluded to, the parallelism is either 

 to a great extent shown in external characters, or in 

 structures which are easily modified ; but, as we shall 

 indicate in the sequel, in other cases it affects deeply- 

 seated structures ; and its inducing cause is then very 

 difficult to surmise on any of the ordinarily accepted 

 doctrines of evolution. It will, moreover, be ob\aous that 

 the acceptation of parallelism in development — and ac- 

 cepted to a certain extent it must undoubtedly be — throws 

 a new difficulty in the interpretation of the affinities of 

 animals, since before saying that an identity in some 

 structural feature between the members of any two groups 

 indicates their relationship, we have first of all to deter- 

 mine whether such similarity of structure is due to 

 parallelism, or is inherited from a common ancestral type. 

 As is so generally the case when any new theory is started, 

 a host of enthusiastic writers have welcomed parallelism 

 with acclamation, and have attempted to apply it in a 

 number of instances where there is not at present sufficient 

 evidence of its existence. We have been told, for instance, 

 that the American monkeys have no relationship with 

 their reputed cousins of the Old World ; that whalebone 

 whales are not allied to the sperm-whale and dolphins ; 

 that cats have no Icinship with civets or other modern 

 carnivores ; and that the egg-laying mammals have been 

 evolved from a reptilian or amphibian stock, quite inde- 

 pendently of ail other members of the mammalian class — 

 their resemblances being solely due to parallelism. At 

 present, we confess, we are totally unable to accejit any of 

 these conclusions, and we think it somewhat improbable 

 that if the members of either of the above-mentioned 

 pairs of groups had an independent origin, they would 

 have presented such a similarity, both externally and 

 internally, as we find to be the case. Still, however, it 

 must be borne in mind that there is a considerable amount 

 of evidence that the modern horses of the Old World and 

 their extinct cousins of the New have been independently 

 derived from earlier horse-like animals ; and if this be 

 confirmed, it will be one of the most remarkable known 

 instances of parallelism, and will tend to show that many 

 others may exist. Turning from these more or less 

 problematical cases, we proceed to notice si'iiatim certain 

 well-marked instances of parallel development as to the 

 existence of which there can be no doubt ; commencing 

 with those displayed in external features, and then referring 

 to such as are more deeply seated. 



As regards external resemblances, it is, of course — and 

 more especially so far as the lower animals are concerned 

 — not always easy to distinguish between parallelism and 

 mimicry. The above-mentioned instances of mole-like and 

 hedgehog-like animals belong, however, clearly to the 

 former category. The external resemblance existing 

 between swifts and swallows, which have no sort of 

 relationship to one another, likewise conies under the 

 same head. Another striking instance is to be found 

 in the assumption of a snake-like form, accompanied by 



