ANATOMICAL PROBLEMS BEARING UPON EVOLUTION. 11 



necessarily the essential plan of its organisation. It is a 

 process liable to be controlled, modified, and curtailed by 

 other causes, and by the action of other principles in the 

 growth of the animal. It is a valuable guide up to a 

 certain point, but other principles have to be taken into 

 account as well, in making a wide or general comparison 

 of even vertebrates alone. Segmentation is a factor in 

 organic growth which is utilised as far as is needed, but 

 the extent to which it is carried varies extraordinarily in 

 different animals and in parts of the same animal. The 

 process of segmentation moreover is superadded to the 

 still more fundamental style of architecture, the longi- 

 tudinal tubular arrangement of the essential organs of 

 the body. 



Embryological Difficulties. 



The problem of evolution also faces us in the study 

 of the growth and development of an animal. Of course 

 among vertebrates and more particularly mammals, we 

 are dealing with structures highly specialised and compli- 

 cated. But the problem is essentially the same as in 

 simpler forms, — to understand what determines the 

 differentiation of the cellular constituents of the organism, 

 because ultimately this cellular differentiation is the 

 cause of the individual and specific characters of a 

 particular form — the leopard's spots and the Ethiopian's 

 skin. 



A complex organism is buill up of organs. Its 

 organs are composed of tissues, and its tissues of re] Is and 

 their derivatives. In the skeleton there are a series of 

 structures which are obviously, from their size, durability 

 and importance in the animal economy, of the greatest use 

 in the study of comparative anatomy. In comparing one 

 animal with another, or making out the homologies of 

 their structure, a basis of comparison musl be taken either 



