72 REPRODUCTION AND LIFE HISTORY. 
is narrowed. The rabbit begins like a Protozoon as a 
single cell; after a while it may be compared to the 
young stage of a very simple vertebrate; afterwards, to 
the young stage of a reptile; afterwards, to the young 
stage of almost any mammal; afterwards, to the young 
stage of almost any rodent; eventually it becomes un- 
mistakably a young rabbit. 
Herbert Spencer expressed the same idea, by saying that 
the progress of development is from homogeneous to 
heterogeneous, through steps in which the individual 
history is parallel to that of the race. But Haeckel has 
illustrated the idea more vividly, and summed it up more 
tersely, than any other naturalist. His ‘fundamental 
biogenetic law” reads: “Ontogeny, or the development 
of the individual, is a shortened recapitulation of phylogeny, 
or the evolution of the race.” 
It is hardly necessary to say that the young mammal is 
never like a worm, or a fish, or a reptile. It is at most 
like the embryonic stages of these, and it may also be 
noticed that, as our knowledge is becoming more intimate, 
the individual peculiarities of different embryos are be- 
coming more evident. But this need not lead us to deny 
the general resemblance. 
Moreover, the individual life history is much shortened 
compared with that of the race. Not merely does the one 
take place in days, while the other has progressed through 
ages, but stages are often skipped, and short cuts are dis- 
covered. And again, many young animals, especially those 
“larvee” which are very unlike their parents, often exhibit 
characters which are secondary adaptations to modes of 
life of which their ancestors had probably no experience. 
In short, the individual’s recapitulation of racial history is 
general, but not precise. It is seen rather in the stages 
in the development of organs (organogenesis) than in the 
development of the organism as a whole. 
(4) Organic continuity between generations.—Heredity.— 
Everyone knows that like tends to beget like, that offspring 
resemble their parents and their ancestors. Not only 
are the general characteristics reproduced, but minute 
features, idiosyncrasies, and pathological conditions, inborn 
in the parents, may recur in the offspring. 
