KLEINENBERG ON DEVELOPMENT OF LOPADORHYNOHUS. 543 



joined to those already present to form a mass, the component 

 units of which, although unlike, subserve the performance of 

 some one physiological function (e. g. glandular and pavement 

 epithelia, connective tissue, muscle-fibres) ; the whole aggre- 

 gate forms a single organ, and the new differentiations only 

 serve to increase its efficiency. In other instances groups of 

 cells, subserving a special function, will induce changes in 

 adjoining cells, which changes are the first step towards the 

 establishment of new tissues and organs. In every case change 

 of function is the guiding principle in the formation of fresh 

 tissues and organs. 



It is a mistake to suppose that a new organ is formed by the 

 gradual growth and change of a pre-existing organ. Since 

 Darwin showed that species through gradual modification are 

 transformed into new species, it has been generally believed 

 that organs similarly may go through slow series of uninter- 

 rupted modifications, the final result of which is the transfor- 

 mation of that organ into some other organ having an analogous 

 though not wholly similar function, or even into some wholly 

 different organ with entirely different functions. The phylo- 

 genetic and ontogenetic changes of the gill-slits in the Verte- 

 brata are familiar instances of this supposed progressive 

 modification of a single organ. As a matter of fact this process 

 of direct change has seldom, if ever, taken place. In animals 

 of simple organisation tissues and organs are formed to which 

 special functions are appropriate. Their functional activity is, 

 as it were, a disturbing elemeut in the organism ; it induces 

 changes in neighbouring tissues, and gives the signal for new 

 specialisations in them. The functional activities of the newly 

 specialised tissues must always bear some relation to the 

 function of the organ which determined their origin, and must 

 either support or modify their action. The newly-formed tissues 

 again affect the organism, their importance increases, and they 

 may in their turn give rise to fresh tissues. Finally, they may 

 become so important that they outweigh in functional import- 

 ance the organ to which their origin was due; they then take 

 its place, and the latter dwindles till finally it may disappear 



VOL. XXVI II, PART 4, NEW SEtt. O O 



