THE PRINCIPLES OF BIOLOGY. 
101 
embryo of the mammal it serves no such purpose ; it is either 
reabsorbed, as in the implacental mammals, or it becomes 
placental—that is, it serves as a connection between the 
embryo and the parent animal for the admission of nutritive 
matter. 
The development by different stages is called indirect 
development. But in some cases the germ forms itself almost 
directly into the form of the mature organism ; such develop¬ 
ment is called direct. Direct development is found among the 
spiders, some gastropods, and most of the lowest classes of 
animals, while the indirect development characterises the 
higher classes. On the theory of evolution the development 
of each organ must be a sort of condensed repetition of the 
development of that organ in the ancestral races, and must 
therefore be indirect originally. The direct method must 
therefore have been substituted for the indirect one wher¬ 
ever it occurs. To find the reason of this substitution, Mr. 
Spencer compares the embryonic development with the 
formation of social agencies. He shows that now such 
agencies as colonies, towns, mercantile associations, &c., 
are formed directly, while formerly they were formed indi¬ 
rectly and by degrees. The reason is that such agencies 
having been long established or being very prevalent, people’s 
ideas have become changed, and they at once arrange them¬ 
selves according to a finished plan instead of going through 
all the stages of development. In like manner we can 
explain the substitution of direct for indirect development; 
as each organism consists of physiological units, these must 
be altered by external forces acting on and modifying the 
whole organism. 
If these external forces cease to act, no modifications 
will take place, and a complete harmony between the action 
and re-action of the organism and its units will be established 
in time—that is, the units will be so modified as to shape them¬ 
selves at once into the form of the organ or organism, and we 
have direct development; but continued action of external 
forces will prevent this harmony beiug established, and each 
modification of the organism, causing a modification in the 
units, will be inherited and reproduced during the development 
of the individual. From this we conclude that we ought to 
find development direct where the conditions have been most 
constant, and indirect where the modifications through which 
an organism has passed during its evolution from its ancestors 
have been numerous and great. This is what we find, as the 
low types which possess direct development show by their 
inferiority that since their first production they have not 
