248 AN INTRODUCTION TO MODERN GENETICS 



no question in this case of a strictly palingenetic type of evolution, in 

 fact, new characters are said to appear in the first formed thecae of a 

 colony. Nor is there any evidence of a circular evolutionary course. But 

 the whole of graptolite evolution can, it is claimed, be described in terms 

 of a very few progressive series of changes, of which the most impor- 

 tant are (i) reduction of number of branches; (2) change from the 

 pendent form to a form with branches sticking upwards; (3) elaboration 

 of the shape of the thecae. 



Evolution normally proceeded in a definite direction along each of 

 these Unes, although some examples of reversal of direction, i.e. re- 

 gression, are known. The progress along each of the three different 

 lines was to a large extent independent; one phylogenetic stock might 

 rapidly reduce the number of its branches while retaining simple 

 thecae, another might develop elaborate thecae while still having a 

 large nimiber of branches. Each of the series of changes is often com- 

 pared with the trends discussed above, and is often spoken of as a 

 trend; but the graptolite trends on the whole took much longer to run 

 through than those of the Gryphaeas for example, and the evidence 

 for them comes from the seriation of comparatively rare specimens, 

 and not from collections so numerous that we can follow a continuous 

 process of change. There is therefore room for errors of interpretation; 

 the orderly programme of evolution may perhaps represent the neatness 

 of the systematists' mind rather than anything to do with the graptoHtes 

 themselves. While such a doubt is still possible, it is better to distin- 

 guish between the large-scale and more doubtful programme evolution 

 and the small-scale imdoubted trends. Further palaeontological re- 

 search is necessary before we can judge how far such doubts are justified. 

 Meanwhile the geneticist should note the alleged phenomenon of 

 programme evolution as one which may require his attention; at 

 present genetics has very little to say about evolution over such long 

 periods of time. 



2. The Sudden Origin of Species: Age and Area 



Examples have been given (p. 256) of the sudden origin of new 

 species by the formation of tetraploids in plant hybrids, both in nature 

 and in the laboratory. There is other evidence that many plant species, 

 which may not always be polyploids, have a sudden origin. Willis,^ 

 working on the flora of Ceylon, pointed out that a rare species tends to 

 be confined to a very small area in a certain locality. He showed that 



1 Willis 1922. 



