REVIEWS 171 



phenomena. Thus Hydra leads to symbiosis and by a reference to Obelia to 

 polymorphism : Tcenia is used to introduce parasitism, protective adaptations 

 against parasites, and immunity. The first chapter is a general introduction to 

 the vital phenomena exhibited by protoplasm and the last deals with a number of 

 problems related to evolution. 



The illustrations are throughout good, many of them taken from Sedgwick and 

 Wilson's General Biology, to which the book forms a suitable companion, a 

 number of others appearing here for the first time, and some of these, e.g. two of 

 the earthworm, are very useful. 



It is a well-planned work and one that can be read with profit and enjoyment 

 by all students of biology. 



C. H. O'D. 



The Mechanism of Mendelian Heredity. By T. H. Morgan, A. H. Sturte- 

 vant, H. J. Muller, and C. B. Bridges. [Pp. xiii + 262, with a frontis- 

 piece and 64 illustrations.] (London: Constable & Co., 191 5. Price \2s. 

 net.) 



PROF. T. H. Morgan is well known as a supporter of the biological teachings 

 grouped together under the name of " Mendelism," and his researches with those 

 of his co-workers have much enriched our knowledge of the phenomena of 

 heredity. Most of this work has been carried out on the fruit-fly Drosophila 

 auipelophila, which has consequently become as much a classic type as the 

 CEnothera of De Vries or the peas of Mendel himself. Drosophila possesses a 

 number of advantages for the purposes of investigation : it breeds freely and 

 rapidly, is easily manipulated, and, what is extremely useful, entails the minimum 

 of space and expense. These points made it possible for more than one hundred 

 characters to be studied within a comparatively short period of time. It was 

 found that these characters tended to be inherited in four groups : two of which 

 were large and one very small. This becomes the more interesting when we find 

 that the somatic number of chromosomes is four pairs in both male and female, 

 and that they consist of two pairs of large, one pair of median, and one pair of 

 quite small chromosomes. The smallness of this diploid chromosome number 

 much facilitates the cytological study of the germ-cells. The ordinary phenomena 

 of Mendelian inheritance are well illustrated by a number of characters in the fly. 



Two of the most interesting phenomena are linkage and crossing over. To 

 quote : " Whenever the two factors remain together in the same chromosome 

 there will be formed equal numbers of gametes containing the two factors, and of 

 gametes containing the normal allelomorphs of the two factors. But if pieces of 

 homologous chromosomes are interchanged, then some of the gametes will con- 

 tain one of the factors in question, and an equal number will contain the other 

 factor. The process of interchange between chromosomes is called crossing over ; 

 the tendency of factors to stay together is called linkage." Reasons are given for 

 supposing that "the percentage of crossing over is an expression of the distance of 

 the factors from each other," and on these lines an interesting diagram of the 

 relative distribution of the factors in the chromosomes is constructed. 



Useful chapters on sex inheritance, the constitution and distribution of the 

 chromosomes, multiple allelomorphs and multiple factors follow. The concluding 

 chapter treats of the " Factorial Hypothesis " and summarises the authors' views 

 on Mendelian factors and the part they play in heredity. Two theories prevalent 

 among Mendelians in this country, namely, the presence and absence theory and 

 the theory of inhibitors, are criticised strongly, and a very good case against both 



