82 SECTIONAL ADDRESSES 



of the gill and other structures on the morphological left side of the 

 mantle cavity, and by a tightening up of the twist in some parts of the 

 nervous system which is the despair of the student who is dissecting his 

 first gastropod types. Though these changes cannot be followed in 

 fossils, they are indirectly reflected by certain features in the shell such 

 as the slits and siphons of the mouth margin, or they are associated with 

 types of shell such as those of Pleurotomaria, Littorina, Cerithium, and 

 thus throw some sidelights upon the geological history of these anatomical 

 changes. Hitherto no cases of unit characters appearing deuterogeneti- 

 cally have been detected. 



Trend characters, on the other hand, arise either coenogenetically or 

 deuterogenetically and proceed proterogenetically or tachygenetically 

 towards later or earlier stages in life-history respectively in successive 

 generations. In both cases the advancement of the trend is accompanied 

 by a displacement of homologous characters — -that is to say, characters 

 situated in or on homologous parts. In the former case displacement is 

 towards late life and culminates in the disappearance of the older characters 

 at the end of life. In the latter displacement is towards early life and 

 ends, usually at the junction of the embryonic and neanic phases, in the 

 elimination of these characters. This phenomenon has long been known 

 as lipopalingenesis. 



A priori there seems to be no reason why both types of development 

 should not proceed simultaneously in a series of solitary organisms for 

 different sets of characters, but hitherto I have failed to detect any 

 examples of this. That they may occur in sequence or simultaneously in 

 closely allied organisms is well illustrated by the history of certain colonial 

 forms, more especially the graptolites. As long ago as 1923 Miss EUes 

 in her illuminating presidential address to this Section drew attention to 

 a manifestation of the phenomenon now referred to under the heading 

 proterogenesis. Speaking more specifically of thecae she says, ' it may 

 be noted at this point that all progressive development (anagenesis) 

 occurs at the proximal and, therefore, youthful region of the rhabdosoma.' 

 But, she goes on to observe, ' when retrogression (catagenesis) occurs it 

 is in this same proximal region that the signs of former elaboration are 

 retained.' In other words, the retrogressive changes proceed according 

 to recapitulatory principles. 



Miss EUes's statements have been recently amplified and fully illustrated 

 in a paper by Bulman in which he gives a useful and suggestive summary 

 of the present state of knowledge of the evolution of graptolites. Only 

 two examples, selected from his account, may be mentioned here. The 

 first is a progressive series, viz. Monograptus raitzhainensis-Rastrites 

 maximus. Here the thecae, that are tubular and distinctly separated from 

 one another, appear for the first time at the proximal end of the stipe of 

 the early member of the series. In later representatives the thecae of 

 this type extend progressively along the whole length of the stipe and 

 thus clearly exemplify the principle of proterogenesis. Nevertheless it 

 should be noted that the degree of elongation of the thecae increases " 

 towards the distal end and that the degree thus attained in early types 

 passes backwards towards the proximal end in later types. In this stock 



