No two alike

There is an enormous number of plant and animal species in the living world, and you can tell one from another just by looking. This chapter is about why that is possible — and about why it is not the whole story.

Telling one species from another

You can separate the living things of one species from those of another by observing their external features. That ability comes from each species having characteristics of its own, peculiar to it, that no other species has in the same combination.

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Figure 20.1 — a group of people from different regions of the world, showing the range of human appearance.

And within one species too

But a species having its own characteristics does not make every member of it identical. No two people have exactly the same body features; there are many differences among them. The same is true of every species you can look at closely.

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Figure 20.2 — a grid of domestic cats of different breeds and a grid of parrots of different species and colours, showing variety within the animals people keep.

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Figure 20.3 — roses and orchids in a range of colours and sizes, showing variety among garden flowers of the same kind.

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Figure 20.4 — a basket of tomatoes of several varieties and a basket of brinjals of several varieties, differing in colour, size and shape.

Two questions, not one

Why is a cat recognisably a cat? And why is no cat quite like the next one? The rest of this chapter answers both with the same machinery — the second question is the more interesting one, and it is where Mendel started.