June 8, 1900.] 



SCIENCE. 



.885 



that any of them will be the Magnum Bo- 

 num with all its properties of keeping, resist- 

 ing disease and so forth ? Not a bit of it. 

 The probability is, that not one of your 

 seedling plants will exactly reproduce the 

 parents ; they will all be different. Again, 

 take the apple; if you sow the seed of a 

 Blenheim Orange and raise young apple- 

 trees, you will not get a Blenheim Orange. 

 All your plants will be different, and prob- 

 ably not one will give you apples with the 

 peculiar excellence of the parent. If you 

 want to propagate your Blenheim Orange 

 and increase the number of your trees, you 

 must proceed by grafting or by striking cut- 

 tings, which are the methods by which such 

 a tree may be asexually reproduced. And 

 so on. Examples might be multiplied in- 

 definitely. Every horticulturist knows 

 that variety characterizes seedlings, i. e., 

 sexual offspring, whereas identity is found 

 in slips, grafts and offsets, i. e., in asexual 

 offspring ; and that if you want to get a 

 new plant you must sow seeds, while if you 

 want to increase your stock of an old one 

 you must strike cuttings, plant tubers or 

 proceed in some analogous manner. 



An apparent exception to this rule is 

 afforded by so-called bud variation, but it 

 is not certain that this is really an exception. 

 In so far as these bud variations are not of 

 the nature of acquired variations produced 

 by a change of external conditions, and dis- 

 appearing as soon as the old conditions are 

 renewed, they are probably stages in the 

 growth and development of the organism. 

 That is to say, they are of the same nature 

 as those peculiarities in animals which ap- 

 pear at a particular time of life, such as a 

 single lock of hair of a different color from 

 the rest of the hair,* the change in color of 

 hair with growth, t the appearance of insan- 

 ity or of epilepsy at a particular age. There 



* Darwin, Variation, Vol. I., p. 449. 

 t As an example I may refer to the Himalayan rab- 

 bit ; Darwin, Variation, Vol. I., p. 114. 



is nothing more remarkable in a single bud 

 on a tree departing from the usual charac- 

 ter at a particular time of life, than in a 

 particular hair of a mammal doing the 

 same thing. 



We have seen that, speaking broadly, 

 genetic variation is connected with sexual 

 reproduction, and it becomes necessary to 

 examine this mode of reproduction a little 

 more fully. What is the essence of sex- 

 ual reproduction, and how does it differ 

 from asexual ? What I am now going to 

 say applies generally to the phenomenon 

 whether it occurs in plants or animals. Sex- 

 ual reproduction is generally carried on by 

 the co-operation of two distinct individu- 

 als — these are called the male and female 

 respectively. They produce, by a process 

 of unequal fission which takes place at a 

 part of their body, called the reproductive 

 gland, a small living organism called the 

 reproductive cell. The reproductive cell 

 produced by the male is called in animals 

 the spermatazoon, and is different in form 

 from the corresponding cell produced by the 

 female, and called in animals the ovum. 

 The object with which these two organisms 

 are produced is to fuse with one another 

 and give rise to one resultant uninucleated 

 organism or cell, which we may call the 

 zygote. This process of fusion between the 

 two kinds of reproductive cells, which are 

 termed gametes, is called conjugation. The 

 difference in structure between the male 

 and female gamete is a matter of secondary 

 importance only, and is connected with the 

 primary function of coming into contact 

 and fusing. The same may be said with 

 regard to the so-called sexual differences of 

 the parents of the two kinds of gametes, 

 and to the powerful instincts which regu- 

 late their action. The conjugation of the 

 male and female gamete, or the fertilization 

 of the ovum, as it is sometimes called, con- 

 sists in the fusion of two distinct masses of 

 protoplasm which are nearly always pro- 



