5 1 2 Retrospective Criticism. 



state from the margin of the mantle, and afterwards becomes dry. If the 

 cold is very severe, they retreat deeper in the shell, and return very frequently 

 to the operculum, adding one cover after another till the whole is sufficient to 

 keep out the cold. These additional covers are much thinner than the outer 

 one which was first made. They pass the winter under the earth, or in the 

 dust and rubbish of hollow trees. Their eggs are about the size of green peas, 

 and are deposited in a hole in the ground, and amount generally to two or 

 three dozen. In the South of Europe, viz., in Italy, France, and England, and 

 even in Asia, the rough vineyard snail (H. aspera) is so abundant, that it is 

 not only eaten, but soup is made from it for diseases of the chest. It is rather 

 more than an inch in diameter, is rough, and has brown and gray spotted 

 bands with a white opening. (Allgemeine Naturgeschichte fur alle St'dnde, by 

 Professor Oken, vol. ii. p. 421.) 



Art. III. Retrospective Criticism. 



The Suburban Horticulturist. — [The following notes, by Mr. Lymburn, will 

 be perused with interest and advantage by all our readers. To render them 

 clearly understood, we have prefixed the passages in the Suburban Horticul- 

 turist to which they apply.] 



" 9. The next point of analogy between plants and animals which it may be 

 useful to notice is that between the lungs and the leaves. An animal can no 

 more live without its lungs than without its stomach. The stomach, as we. 

 have seen, is necessary for turning the food into chyle, and the lungs for 

 turning that chyle into blood. Now, a plant can no more live and grow 

 without leaves, than an animal can without lungs. The use of the lungs is 

 to expose the chyle to the action of the air, which they decompose, so that 

 its oxygen may unite with the chyle, and thus change it into blood. The 

 leaves of plants, which act to them as lungs, not only decompose air, but 

 light, in the process of elaborating the sap ; and hence plants can no more 

 live without light than without air or food, as light is necessary to turn their 

 food into sap, or, in other words, to bring it into the proper state for afford- 

 ing them nourishment. Hence, in the culture of plants, the great importance 

 of solar light. An important difference, however, between the circulation of 

 the sap in vegetables and that of the blood in animals is, that the former have 

 no heart." 



In comparing plants with animals, the leaves can only be compared to 

 lungs ; and, similarly to lungs, it is true, they aerate the sap, and imbibe 

 oxygen, as the lungs do to the blood : but, when we carry the comparison 

 further, we find that not only do the leaves imbibe oxygen, but they also, by 

 imbibing the chemical power of the light, decompose carbonic acid, absorbing 

 the carbon, and setting the oxygen free. This is a power which has never 

 been ascribed to lungs ; and, as the chemical power absorbed probably acts in 

 other ways on the sap presented (see 124.), though it is difficult to discriminate 

 between organic secretion of particular organs and the chemical power of 

 light, it has been by many eminent physiologists called digestion. Compara- 

 tive physiology is valuable as assisting us to understand more readily what 

 we are ignorant of, by comparing it with what we are already acquainted with. 

 It is necessary to know the functions which the different organs perform before 

 we can estimate their value, or know the necessity of supplying them with 

 proper food ; and the more we can simplify the subject, by classifying one 

 organ in one organised being with one destined to a similar purpose in another, 

 we the more readily arrive at a general knowledge of the whole. There are 

 many difficulties, however, in comparative physiology ; and the proper class 

 of organs to which leaves may belong seems one of the principal stumbling- 

 blocks. 



