riorly necessary to determine better, inasmuch as oflfering an inter- 

 mediate generation, viz. betvveen that proper to tlie Monotremata, 

 and a third sort, that of the Ovovivipara, (that is to say of the Skarks 

 and Rays,) the eggs of vvhich are hatched either within or without 

 the body of the mother, they \vould furnish facts of the šame rank as 

 thoseof the vipers and olher snakes, and vvould not ofFer such im- 

 portant characteristic dift'erences bet\veen all these animals,as have 

 hitherto been uniformly believed to exist. I refrain from proceeding 

 further in order not to overpass the bouudaries of analogies and of 

 truth ; but it miglit happen that the objection proposed by Von Baer 

 should lead to this result; not that the Monotremata should be throvvn 

 back into the centre of the Mammalia, but that the Cetacea should 

 be separated from among them. The affinity of structure, if it be 

 such as tlie German physiologist announces, raay lead to an idea 

 that the mode of nutrition which I have sketched for the Monotre- 

 mata may be eąualiy adapted to the Cetacea, Forinerly one mode 

 onlv was known, and it \vas supposed a priori that the Cetacea mušt 

 have passed through it. At all events it is necessary to revise the 

 doctrine of the nutrition o( ihejcelus of Cetacea. 



" Secondly, Mr. Ovven points out the contradictoriness of*my two 

 opinions in two papers published at an interval of less than a month, 

 and this is fair play in his capacity of critic. Nevertheless I had 

 scarceiy touched on the fact relative to the egg-shells in my first 

 paper, proposing to return to it again. This 1 actually did some 

 weeks aftervvards, when I conccived a system complele in itself, well 

 connected, opening out nevv views to researeh, and of vvhich 1 frankly 

 declare that I had not the sniallest idea a i'ew days before I became 

 attached to it. Let it not, however, be believed that I prtsent either 

 my old or my ne\v conjectures as facts, the solidity of which I de- 

 cidediy maintain. In the absence of facts, I venture to recur to pre- 

 sumptions, which may become motives for researeh ; but if I calcu- 

 late certain probabilities, I merely desire to have applied to them 

 the criterion of observation. 1 know w'ell that the mind of no man 

 is endowed with the faculty of imagining vvith regard to substantiai 

 bodies, of distinctly conceiving the idea of a form. What has been 

 Eeen of this kind is thenceforth knovvn. Seriously admitting the truth 

 of this proposition, I merely wish to play a useful part, restricting 

 myself to the duties of a naturalist having the privilege of age, con- 

 fident in the experience of ancient studies, and acquainted with the 

 possible extcnt of the diversities of the acts of nature, in order to 

 assist observers less practised than myself in the study of natūrai 

 history, so that if there should exist in the most distant part of the 

 globė, organic conditions Hhich we are interested in becoming im- 

 mediately acąuainted vvith, I may say to them ' There is a chance 

 that it is A, or B, or C ; see vvhat is the fact; instruct us vvith re- 

 gard to it.' 



" Thirdly, The monotrematic glands follovv the pliases of the de- 

 velopment of the sexual apparatus : likę the mammary, they form 

 part of it, being large only in the females. To this I ansvver that it is 

 presuming too niuch vvith regard to the resources of nature, (vvhich 



