242 AN INTRODUCTION TO MODERN GENETICS 



of change which has been called a "trend." The evidence for the 

 small-scale trends is, however, considerably more compelling than that 

 for the large-scale programme evolution. We will discuss the two 

 phenomena separately, taking the small-scale trends first. 



(a) Trends. — One of the classical examples of evolution in trends is 

 provided by the Gryphaeas of the Jurassic.^ During that period several 

 different stocks of oyster-like Lamellibranchs belonging to the genus 

 Ostrea independently evolved along a certain line of change which 

 converted them into forms classified in the genus Gryphaea. The 

 essential change was the development of a very thick shell with highly- 

 curved mnbones. At any given time, represented by a certain geological 

 horizon, the population of Gryphaeas belonging to one of these stocks 

 shows a certain amount of variation in these characters. Part of the 

 variation is due to age differences; old specimens have thicker shells 

 than young ones, and are also larger in area. Variation due to age 

 differences can therefore be eliminated by considering only shells of 

 the same size. Within a group of shells of the same size, the variation is 

 distributed in the normal frequency curve. In a higher, i.e. later, 

 horizon, it will be found that the whole of this curve of variation is 

 shifted towards greater thickness and curvature. By such insensible 

 changes of frequency distribution the population gradually comes to 

 consist of forms which would be classified in a different genus. Clearly 

 the usual concept of species caimot be appUed to such a case, and 

 palaeontologists usually speak of such assemblages as a gens. 



In the Gryphaeas, the evolutionary changes seem at first sight to 

 carry forward in a general way the developmental changes which occur 

 in each life history. We can regard an early, flat form as consisting of a 

 short piece of a spirally coiled shell, while the later form with a curved 

 umbo simply consists of a larger part of the same spiral and is derived 

 from the flat form by mere continuation in its spiral growth. Phenomena 

 in which a later evolutionary form is produced by continuing a develop- 

 mental process which stopped short in its ancestors is known as palin- 

 genesis, and we shall see that such a process is described as occurring 

 in the large-scale programme evolutions which will be discussed later. 

 It is therefore of importance to note that the apparent palingenesis of 

 the imibonal curvature of the Gryphaeas will not bear critical examina- 

 tion. The evolutionary sequence does not involve merely the develop- 

 ment of a larger amount of the same spiral, but the actual tightness of 

 the coil increases in the later forms. 



1 Tnieman 1922, 1930, 



