32 EXPERIMENTAL EVOLUTION LECT.. 



down to the fishes themselves, as, for instance, in its 

 temporary branchial slits and arches, in the primitive 

 circulatory apparatus of the earlier stages of develop- 

 ment, in the various forms which the central nervous 

 system presents at various periods. The evolution of 

 the circulatory apparatus is wonderful. At the begin- 

 ning, during the first hour of evolution, the heart is a 

 mere tube or bulb, exactly similar to the heart of the 

 ascidians. Through some modifications, it then pre- 

 sents the typical aspect of the heart of mud fishes 

 or Dipnoi'. Later on, we meet with the condition 

 persistent in adult amphibians ; then follows a stage 

 which corresponds to that of reptiles, and finally 

 the heart corresponds to that of birds and mammals. 

 The same process is to be seen in the evolution of 

 the principal blood-vessels which are attached to the 

 central organ of circulation, and the same stages are 

 successively gone through. Classical as these facts 

 may be, they may be briefly recalled, as their 

 signification is of great weight. All fishes, it 

 is well known, have a number of gill-arches on 

 each side. In the amphioxus or lancelet, the 

 lowest of known fish-like forms, there are very 

 numerous slits, doubtfully homologous with those 

 of true fishes, which have seven, five, four, or three. 

 Their use is quite clear : the blood flows through the 

 arches and the fringes they support, and thus be- 



