472 the president's address. 



indeed are in all probability descended from annelid ancestor 

 The common crayfish, for example, is made up of nineteen, or, 

 according to some authorities, twenty segments, each having its 

 own pair of limbs or appendages, all of which can be readily 

 derived from one and the same common type of structure. In 

 the arthropods, however, we find the process of integration 

 carried much farther than it is in the annelids. Any ordinary 

 insect, as you know, shows a well-marked differentiation into 

 head, thorax and abdomen, each of which is composed of a 

 number of segments which co-operate in the fulfilment of some 

 common function, or rather of many common functions. There 

 is not only differentiation and division of labour between individual 

 segments, but the segments are grouped so as to perform their 

 functions more advantageously. 



It is precisely the same in the highest phylum of the animal 

 kingdom, the Vertebrata. These are all metamerically segmented 

 animals, derived in all probability from some metamerically 

 segmented, worm-like ancestral form. The process of integration 

 has gone so far, however, that but few indications are left, 

 externally at any rate, of their origin ; though we see abundant 

 traces of serial metamerism in their internal organisation, as for 

 example in the segmented vertebral column and the segmentally 

 arranged cranial and spinal nerves. In the early stages of develop- 

 ment the metameric segmentation is much more obvious and 

 cannot possibly be overlooked. 



It may seem absurd enough to the layman to say that the 

 human head is made up of at least twelve segments, each of 

 which corresponds to a complete individual in some remote 

 ancestral linear colony, but the statement is in all probability 

 strictly true. 



In the main line of evolution of the animal kingdom, then, 

 we can recognise three very distinct grades or orders of indi- 

 viduality from the morphological point of view. First, the single 

 cell, as in the Protozoa ; second, the simple multicellular type, as 

 in the Coelenterata and the majority of the flatworms, and 

 third, the metamerically segmented type, as in the annelid 

 worms, the arthropods and the vertebrates ; and each succeeding 

 higher grade has been derived from the one below it through the 

 process of colony formation, followed by differentiation, division 

 of labour and integration. 



