Here's what popular dog breeds looked like before and after 100 years of breeding

Dogs have been our furry companions for thousands of years, but they didn't always look the way they do today.

Many well-known breeds have changed a lot physically in the last century, thanks to humans.

By identifying specific traits — such as size, coat color, and demeanor — and allowing only those animals to mate, we've created at least 167 different "breeds," or groups of dogs with unique physical and mental characteristics. Still, they're all part of the same species.

The Science of Dogs blog put together a side-by-side comparison of several popular breeds from the 1915 book "Dogs of All Nations" by Walter Esplin Mason, showing what they look like today.

Here are some of the dogs from that list, plus a couple more we found ourselves.

Tanya Lewis contributed to an earlier version of this post. 

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Bull terrier then

The bull terrier was first recognized as a breed by the American Kennel Club (AKC) in 1885. In 1915, it appears to have been a fit, good-looking dog, with a well-proportioned head and slim torso. "Dogs of All Nations" called it "the embodiment of agility, grace, elegance and determination," and the "gladiator of the canine race."

Bull terrier now

But today, bull terriers are bred to have a football-shaped head and a thick, squat body — a far cry from the lean and handsome dog of 1915.

The AKC now states that the dog's face"should be oval in outline and be filled completely up giving the impression of fullness with a surface devoid of hollows or indentations, i.e., egg shaped."According to Science of Dogs, it also developed extra teeth and a habit of chasing its tail.

English bulldog then

Few dogs have been as artificially shaped by breeding as the English bulldog. In the UK, the dogs were used for bull-baiting— a blood sport where dogs were used to bait and attack bulls — until it became illegal in 1835. In 1915, the bulldog already had some of the characteristic features we see today, like saggy jowls and a squat stance.