LIFE IN PRIMARY PERIOD 227 



are consequently unequal : these are the Aviculida from which 

 the Monomyaria are derived, forms with a single valve- 

 retractor muscle, represented in the Devonian by the Pectens. 

 By the side of these are found Lamellibranchs of the normal 

 type : Anodontopsis, Paracyclas, Amita, etc., reminiscent, 

 to a certain extent, of the forms found in our fresh waters. 



Vertebrates have not yet been encountered in the Cambrian ; 

 but they are represented in the Silurian and the Devonian 

 by Fishes, and also in the Carboniferous by Batrachians, 

 and finally by true Reptiles. This succession would seem to 

 indicate that we have arrived at the point when the evolution 

 of the Vertebrates begins. We know that they must have 

 started in a form analogous to Amphioxus. The nature of 

 the tissues of Amphioxus does not readily lend itself to 

 fossilization, but we have found fossilized Medusae, whose 

 tissues are even softer, and we must not give up all hope of 

 also discovering fossil ancestors of the Vertebrates. After 

 Amphioxus, the simplest Fishes — simplest because the 

 vertebral column consists only of the embryonic dorsal chord — 

 are the Marsipobranchii, of which the Lampreys are typical. 

 Mounting up from these, the evolution of fishes follows a logical 

 order. In my Traite de Zoologie, 1003, I began their history 

 by calling attention to the modelling they had undergone 

 by the pressure of the surrounding water, occasioned chiefly 

 by the sudden movements of their tails in swimming, and to 

 the fact that the number as well as the position of their dorsal 

 fins was due to the tearing of an originally continuous dorsal 

 fin by the currents that were thus formed on the creature's 

 sides. 1 Frederic Houssay has been able to demonstrate 

 this by interesting experiments. He showed that the first 

 stage in the development of a fish's shape is that taken by 

 a cylindrical linen sack filled with a soft paste, when it is drawn 

 horizontally through water or held horizontally in a vase full 

 of v/ater and opposite an aperture through which the water 

 flows out. This form is that described as "la veine inversee ", 

 because it results from the pressure exercised by runnels of 

 water flowing swiftly into the place of the water running out. 

 But if we can thus account for the normal shape of Fish, the 

 explanation does not stand for the special forms they some- 



1 XLIII, 2364 ff. 



