48 PARALLELISM IN DEVELOPMENT. 



horses of the Okl 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 seriatim 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 resemblances existing 

 between swifts and swallows, which have ncj sort of 

 relationship to one another, likewise comes under the same 

 head. Another striking instance is to be found in the 

 assumption of a snake-like form, accompanied by abortion 

 of the limbs, by certain lizards, such as the familiar 

 English "blind worm," it being very doubtful, to our 

 thinking, if this can be explained by mimicry. Still more 

 remarkable is the external resemblance presented by 

 whales and doljjhins to fishes, and by the extinct fish- 

 lizards (Ichthyosaurs) to both ; such resemblances being 

 clearly due to parallel development induced bv the needs 

 of adajstation to a jiurely acjuatic life. If, moreover, it 

 shouldin-ovetha.t the whalebone whales have no connection 

 with the dolphins, we should then have a far more remark- 

 able instance of this feature, and one extending to internal 

 structures as well as to external form. A similar remark 

 will also apply to the case of the true seals and the eared 

 seals, which it has been suggested may be of independent 

 origin. The independent acquisition of wings by birds, 

 bats, and the extinct flying dragons, or pterodactyles, may 

 likewise certainly come under the heading of parallel 

 development, so far as external form and the adaptation 

 to a particular mode of life are concerned ; although here. 



