SECTIONAL TRANSACTIONS.— D. 565 



testicular hormone is physiologically unsound, and the mutation theory furnishes no 

 reason why somatic sexual characters should be influenced by the metabolism of the 

 sexual organs any more than other somatic structures. 



The recent progress of biological knowledge tends to confirm the conception of the 

 adaptive evolution of animals as the expression and result of reactions to the stimuli 

 of the environment, and to justify the conclusion that the genes of the gametes are 

 sooner or later influenced by the modifications of the soma. 



Afternoon. 



Joint Discussion with Section K {q.v.) on A Biological Investigation of 

 British Fresh Waters. 



Tuesday, September 11. 



Dr. G. P. Bidder. — The Interpretation of the Embryology of Sponges. 



The free-swimmiug larvae of sponges do not recapitulate anj' adult ancestral form, 

 their chn.racteristics are due to special adaptations for distribution and to precocious 

 segregation of tissues. There are, however, two colonial Protozoan forms of whose 

 recapitulation there is possibly evidence. (1) There are signs of a free-swimming 

 ancestor which we may call a ' corona,' an eight-celled circle of flagellates resembling 

 the eight-celled protomastigine Bicoeca socialis Lauterborn and the sixteen-celled 

 chrysomonad Cyclonexis. (2) The ' corona ' may have been succeeded by the 

 ' cylindrus,' an open cylindrical tube of 128 or more flagellate cells. These two forms 

 are repeated without apparent embr3'ological advantage in the ontogenies of the 

 sponges and of their descendants. 



Embryology and physiology alike indicate that in tlie ancestral flagellate colony 

 multiplication of cells was by longitudinal fission alone, as in almost all existing 

 flagellates. For the cylindrus we may deduce also a habit of migration of cells to 

 the interior of the tube, resulting first in the formation of a double laj^er of cells, 

 clothing the cylinder with flagella outside and inside. Secondlj', some of the cells 

 permanently lost their flagella and the colony became truly Parazoan, consisting of 

 two different classes of cells. 



Sex has been developed strongly in certain classes of the sponges, but in others 

 the advantages of sex appear to be only attained through the chance, but frequent, 

 fusion of growing colonies. Sex would therefore appear to have been very imperfectly 

 attained in the colonial flagellate ancestor. With sex originated the bilateralism so 

 characteristic of Metazoa. 



Dr. S. L. HoRA. — Animal Ecology of Torrential Streams with special 

 reference to Structural Adaptations. 



There appears to be a growing feeling at the present day that evolution is pre- 

 determined and that adaptation ' is a chance relationship, not a progressive 

 modification towards particular habitat requirements.' This is due to a mis- 

 apprehension. Adaptation signifies correlation of an animal with its habitat and, 

 therefore, for the understanding of this phenomenon a superficial knowledge of animal 

 ecology is not enough. Attempt has been made to classify the unlimited gradations 

 in the environment of the torrential fauna. By referring to the two types of 

 Megalopkrys tadpoles and to the gradual modification and evolution of tiie Sisorid 

 and the Homalopterid fishes it is indicated that evolution is no more than adaptation 

 of organisms to environment. Attention is directed to the different body-forms of 

 brook-inhabitants, to the devices for increasing specific gravity and to the similar 

 mechanisms for producing differential pressure, and it is concluded that similar 

 modifications are directed towards the achievement of similar ends (convergence). 

 The significance of the principles of ' Opportunity Dispersal ' and of ' Cliange of 

 Functions ' is discussed, and it is concluded that evolution proceeds by a continuous 

 adjustment — progressive or retrogressive — of the existing material and that the 

 adaptive structures do not arise de novo. 



