July 1 8, 1878] 



NATURE 



309 



THE GENESIS OF LIMBS 1 



n. 



In those huge marine reptiles of the secondary period, 

 the Ichthyosaurus and the Plesiosaurus, we meet with 

 the most complete serial symmetry between the fore and 

 the hind limbs. In the Plesiosaurus, not only are the 

 bones of the hand and foot completely alike, but the 

 same is the case with those of the upper arm and thigh 

 and those of the fore-arm and leg. 



In the Ichthyosaurus there is a similar resemblance 

 between the fore and hind limbs, with the further curious 

 similarity that in both hand and foot the very numerous 

 digital bones are so disposed as to indicate that the 

 number of the digits exceeded fire. 



When we descend from reptiles to batrachians we again 

 find in the tailed forms a remarkable and exceptional serial 

 homology in the bones and cartilages of the limbs. Thus, 

 through the great descending series of forms from man 

 down to reptiles and efts, we find that there is one funda- 

 mental type of limb. In all such creatures the limbs are 

 never more than four, they are divisible into two pairs, 

 which always possess complete bilateral and more or less 

 clearly marked serial symmetry. This serial symmetry is 

 generally more or less disguised owing to the different 



uses to which the two pairs of limbs are respectively put. 

 Yet whatever such uses and needs may be, whether the 

 limbs be formed simply for locomotion, as in the horse ; 

 for grasping, as in the ape ; for flying, as in the bat and 

 bird ; or for swimming, as in the seal and the large 

 extinct reptiles before referred to, we find the same limb 

 segmentation running through all in both limbs. 



1. An upper limb segment (upper arm and thigh). 



2. A lower limb segment (fore-arm and leg). 



3. A root part of the extremity (wrist and ankle). 



4. A middle part of the extremity (mid-hand and mid- 

 foot). 



5. A terminal part of the extremity (the fingers and 

 toes, i.e., the digits). 



In the great class of fishes an altogether different set 

 of conditions obtains. The parts in fishes which answer 

 to the fore limbs of the creatures hitherto considered, are 

 the pectoral and the ventral fins. In them we find a 

 number of delicate structures, " fin-rays," supported by 

 bones or cartilages which have no obvious, even remote 

 resemblance to the bones or cartilages of the limbs of 

 batrachians, reptiles, birds, or beasts. The pectoral 

 limbs are indeed attached to a shoulder girdle, but the 

 ventral fins are appended only to bones or cartilages 

 which lie amongst the muscles of the ventral surface of 



Fig. 12. — Skeleton of an Ichthyosaurus. 



the body and hardly ever even approach the superin- 

 cumbent vertebral column. In Lophius, indeed, amongst 

 bony fishes, these supporting structures do ascend some- 

 what, and the same is the case in Callorhynchus and 

 Chimcera amongst the cartilaginous fishes. Yet even 

 here these pelvic structures are far from reaching the 

 vertebral column. 



In the fishes, which are in many respects most like the 



Fig. 14. Fig. 15. 



Fig. 14. — ^The two ossa innominata of the Angler-fish (/.(^/Azwi), showing 

 the ascending processes which simulate iha. The fia-rays are attached 

 to the cuter- ventral margin of each os innominatum. 



Fig. 15. — Cartilaginous skeleton of a limb of Ceratodus. (After Giinther.) 

 The large upper piece articulates with the limb root. 



higher animals, i.e., in the sharks and rays, the pectoral 



and ventral fins are not both formed in the same manner, 



but their structure differs considerably in different species. 



The pectoral fin is supported by a number of carti- 



' Continued from p. 284. 



laginous rays which rest upon three basal cartilages lying 

 nearest to the slender girdle and called respectively the 

 Propter ygium, the Mesopterygium, and the Metaptery- 

 gium (see Fig. 17). The ventral fin is smaller, and con- 

 sists of cartilaginous rays appended to a basal cartilage, 

 which is suspended from the pelvic cartilage, to which 

 latter some rays may also be directly attached. 



In one very exceptional form, the ancient triassic fish 

 still surviving in Queensland, Ceratodus, the limb-skeleton 

 is sui generis. It consists of a median longitudinal series 

 of cartilages, whence other smaller cartilages diverge on 

 each side, the whole structure tapering to its distal end. 



We see then, now, the answer to one of our initial 

 questions, " What do our limbs stand for as compared 

 with the bodies of other animals," and may proceed to 

 another of those questions, namely, " Why are our limbs 

 so much alike and yet so different .'' ' ' We form one of 

 a series of creatures with digit-bearing limbs (all verte- 

 brates above fishes), all presenting similar resemblances 

 and differences in varying degrees. The differences 

 between our limbs are manifestly due to difference of 

 function, but the resemblance may be due to one of two 

 causes :— (i) The conservation of a complete serial sym- 

 metry present in the earliest vertebrate types, or (2) 

 The action of internal polar force, tending to educe 

 serial symmetry when such symmetry does not interfere 

 with function. In other words, it may be due either (i) 

 to genetic homogeny, or (2) to intra-organic homoplasy. 



It appears to me impossible that it can be due to the 

 former cause, for even if the first pairs of limbs ever 

 formed were completely similar, which I by no means 

 regard as certain, still the detailed resemblances found in 

 animals high up in the scale cannot have continued unin- 

 terruptedly from such primitive forms. Even if we con- 

 sider that the complete serial homology in the limbs of 

 the tortoise, Chelydra, might be primitive, the limb of 

 Hyrax, Perodicticus, and Nycticebus cannot owe their 

 peculiar serial resemblances * to the survival of primeval 

 conditions, 



» See " Genesis of Species," second edition, p. 201. 



