THE SUDDEN ORIGIN OF NEW TYPES 403 



now endeavour to explain the nature of these " periodical 

 successions of impulses," or "expression-points " of E. D. Cope, 

 by the application of the principle of this paper to some of 

 the main classes of animals. 



In the case of mammals it is obvious that their sudden 

 rise and predominance in the Tertiary epoch, supplanting the 

 reptiles in every branch of life, must have been due to the 

 rapid development of some part or parts of their organisation, 

 which gave them a distinct and overwhelming advantage over 

 their reptilian competitors in the struggle for existence. It 

 is true that the brains of the great Dinosaurians, so far as 

 our evidence goes, are remarkably small in comparison to 

 the size of the animals, and show a low and simple state 

 of organisation ; but the larger and more complex brain does 

 not appear to be the primary factor, although doubtless it 

 was an important element in assuring the predominance of 

 mammals. The primary and determining point of difference 

 between reptiles and mammals obviously lay in the adoption 

 by the latter of mammary glands for nourishing their young; 

 all their other advantages can be derived from this essential 

 feature. The suckling of the young with a perfect food like 

 milk entailed a longer association with the mother, whilst 

 permitting a more gradual attainment of maturity. These 

 factors, together with the greater maternal sacrifice entailed 

 by lactation, would in themselves be sufficient to promote 

 and foster a greater development of brain in mammals than 

 in the case of reptiles, in which the young individual has no 

 other start in life than the yolk within the egg prior to hatching. 

 The acquisition of warm blood and of a hairy covering are 

 probably to be regarded merely as secondary consequences 

 of a general advance in organisation resulting from a novel 

 mode of nutrition of the offspring and the concomitant 

 advantages of a lengthened period of development. 



Now the question naturall}' arises : were the mammary 

 glands absolutely new structures, or were they modified from 

 pre-existing glands ? The latter hypothesis is a priori more 

 probable and more consonant with the usual laws of develop- 

 ment of organs. The mammae form a repetition of similar 

 structures, ranged in series along two convergent lines, 

 extending from the axillae towards the pubic bones. Even in 

 man supernumerary nipples have been shown to occur along 



