3584 * The Zoologist — July, 1873. 



According to this last theory, whether there has been or has not 

 been a material continuity between the lower and higher types, 

 might be open to discussion ; the main difference between it and 

 Darwin's theory is, that it maintains that a material continuity is 

 not necessary, and that the mere laws of necessity (granted a low 

 type of life) and the general conditions of the world are not 

 sufficient to account for that classification of the organic world, 

 which is possible, but that au ideal bond of unity of design is 

 plainly indicated. Whereas Darwin maintains that the bond of 

 unity has been material continuity, produced entirely by the 

 action of the general laws of this planet upon an original simple 

 form of life. Here are Mr. Darwin's views in his own words : — 



" As each species tends by its geometrical ratio of reproduction to increase 

 inordinately in number, and as the modified descendants of each species 

 will be enabled to increase by so much the more as they become diversiBed 

 in habits and structure, so as to be enabled to seize on many and widely 

 different places in the economy of Nature, there will be a constant tendency 

 in natural selection to preserve the most divergent offspring of any one spe- 

 cies. Hence during a loug-continued coarse of modification the slight diffe- 

 rences characteristic of varieties of the same species tend to be augmented 

 into the greater differences characteristic of species of the same genus. 

 New and improved varieties will inevitably supplant and exterminate the 

 older, less improved and intermediate varieties, and thus species are rendered 

 to a large extent defined and distinct objects. Dominant species belonging 

 to the larger groups tend to give birth to new and dominant forms, so that 

 each large gi-oup tends to become still larger, and at the same time more 

 divergent in character. But as all groups cannot thus succeed in increasing 

 in size, for the world would not hold them, the more dominant groups beat 

 the less dominant. This tendency in the large groups to go on increasing 

 in size and diverging in character, together with the almost inevitable con- 

 tingency of much extinctiori, explains the arrangement of all the forms of 

 life in groups subordinate to groups, all within a few great classes which we 

 now see everywhere around us, and which has prevailed throughout all time. 

 This grand fact of the grouping of all organic beings seems to me utterly 

 inexplicable on the theory of creation." 



Now if this grouping has been the result of hereditary connection, 

 how does Darwin account for similar or homologous organs having 

 an independent source ? 



This is the case in point. The marsupial Mammalia form a 

 natural order. No naturalists have ever attempted to separate 



