But some arguments of the greatest weight extend very far.

All the members of whole classes are connected together by a chain of affinities, and all can be classed on the same principle, in groups subordinate to groups.
Fossil remains sometimes tend to fill up very wide intervals between existing orders.
Organs in a rudimentary condition plainly show that an early progenitor had the organ in a fully developed condition, and this in some cases implies an enormous amount of modification in the descendants.
Throughout whole classes various structures are formed on the same pattern, and at a very early age the embryos closely resemble each other.

Therefore I cannot doubt that the theory of descent with modification embraces all the members of the same great class or kingdom.

I believe that animals are descended from at most only four or five progenitors, and plants from an equal or lesser number.
Analogy would lead me one step further, namely, to the belief that all animals and plants are descended from some one prototype.

But analogy may be a deceitful guide.